Loans - Contractual Aging at Amortized Cost in Past Due Loans by Portfolio Segment (Detail) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	$ 2,659,727	$ 2,562,545
Total Financing Receivables	2,658,275	2,560,795
30 to 59 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	3,360	3,769
60 to 89 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	362	712
Greater Than 90 Days [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	2,166	1,117
Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	5,888	5,598
Current [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	2,652,387	2,555,197
Consumer Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	522,416	520,114
Total Financing Receivables	522,930	520,392
Consumer Real Estate [Member] \| 30 to 59 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	2,472	2,533
Consumer Real Estate [Member] \| 60 to 89 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	250	547
Consumer Real Estate [Member] \| Greater Than 90 Days [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	1,387	559
Consumer Real Estate [Member] \| Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	4,109	3,639
Consumer Real Estate [Member] \| Current [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	518,821	516,753
Agricultural Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	222,145	216,401
Total Financing Receivables	221,923	216,137
Agricultural Real Estate [Member] \| 30 to 59 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	336	651
Agricultural Real Estate [Member] \| Greater Than 90 Days [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	376
Agricultural Real Estate [Member] \| Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	712	651
Agricultural Real Estate [Member] \| Current [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	221,211	215,486
Agricultural [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	179,361	152,080
Total Financing Receivables	179,699	152,381
Agricultural [Member] \| 30 to 59 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	109	44
Agricultural [Member] \| Greater Than 90 Days [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	50	79
Agricultural [Member] \| Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	159	123
Agricultural [Member] \| Current [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	179,540	152,258
Commercial Real Estate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	1,355,166	1,310,811
Total Financing Receivables	1,352,690	1,308,461
Commercial Real Estate [Member] \| 30 to 59 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	290	54
Commercial Real Estate [Member] \| 60 to 89 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due		141
Commercial Real Estate [Member] \| Greater Than 90 Days [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	140	360
Commercial Real Estate [Member] \| Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	430	555
Commercial Real Estate [Member] \| Current [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	1,352,260	1,307,906
Commercial and Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Financing Receivables	295,815	274,819
Commercial and Industrial [Member] \| 30 to 59 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due		122
Commercial and Industrial [Member] \| 60 to 89 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	101	5
Commercial and Industrial [Member] \| Greater Than 90 Days [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	197	57
Commercial and Industrial [Member] \| Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	298	184
Commercial and Industrial [Member] \| Current [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	295,517	274,635
Consumer [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	60,469	63,009
Total Financing Receivables	61,132	63,627
Consumer [Member] \| 30 to 59 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	153	365
Consumer [Member] \| 60 to 89 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	11	19
Consumer [Member] \| Greater Than 90 Days [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	16	62
Consumer [Member] \| Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	180	446
Consumer [Member] \| Current [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	60,952	63,181
Other Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	24,086	24,978
Total Financing Receivables	24,086	24,978
Other Loan [Member] \| Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	0
Other Loan [Member] \| Current [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due	$ 24,086	$ 24,978

FARMERS & MERCHANTS BANCORP INC 10-Q