Investments - Available for sale fixed maturities (Details) - USD ($) $ in Millions	Sep. 30, 2025	Jun. 30, 2025	Dec. 31, 2024	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available for sale, amortized cost	$ 10,588		$ 10,687
Fixed maturities, available for sale, allowance for expected credit losses	16	$ 15	34	$ 25	$ 10	$ 12
Available for sale (AFS) fixed maturities	10,518		10,398
Total fixed maturities
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available for sale, amortized cost	10,588		10,687
Fixed maturities, available for sale, allowance for expected credit losses	16		34
Fixed maturities, available for sale, gross unrealized, gains	167		98
Fixed maturities, available for sale, gross unrealized, losses	(221)		(353)
Fixed maturities, available-for-sale securities, accumulated gross unrealized gain (loss), before tax	(54)		(255)
Available for sale (AFS) fixed maturities	10,518		10,398
U.S. government and government agencies
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available for sale, amortized cost	170		176
Fixed maturities, available for sale, allowance for expected credit losses	0		0
Fixed maturities, available for sale, gross unrealized, gains	1		0
Fixed maturities, available for sale, gross unrealized, losses	(1)		(3)
Fixed maturities, available-for-sale securities, accumulated gross unrealized gain (loss), before tax	0		(3)
Available for sale (AFS) fixed maturities	170		173
States, municipalities and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available for sale, amortized cost	877		905
Fixed maturities, available for sale, allowance for expected credit losses	0		0
Fixed maturities, available for sale, gross unrealized, gains	8		3
Fixed maturities, available for sale, gross unrealized, losses	(32)		(49)
Fixed maturities, available-for-sale securities, accumulated gross unrealized gain (loss), before tax	(24)		(46)
Available for sale (AFS) fixed maturities	853		859
Foreign government
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available for sale, amortized cost	249		236
Fixed maturities, available for sale, allowance for expected credit losses	0		0
Fixed maturities, available for sale, gross unrealized, gains	4		2
Fixed maturities, available for sale, gross unrealized, losses	0		(1)
Fixed maturities, available-for-sale securities, accumulated gross unrealized gain (loss), before tax	4		1
Available for sale (AFS) fixed maturities	253		237
Residential MBS
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available for sale, amortized cost	2,388		2,122
Fixed maturities, available for sale, allowance for expected credit losses	1		1
Fixed maturities, available for sale, gross unrealized, gains	37		22
Fixed maturities, available for sale, gross unrealized, losses	(113)		(154)
Fixed maturities, available-for-sale securities, accumulated gross unrealized gain (loss), before tax	(76)		(132)
Available for sale (AFS) fixed maturities	2,311		1,989
Collateralized loan obligations
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available for sale, amortized cost	1,106		1,243
Fixed maturities, available for sale, allowance for expected credit losses	5		4
Fixed maturities, available for sale, gross unrealized, gains	4		10
Fixed maturities, available for sale, gross unrealized, losses	(5)		(12)
Fixed maturities, available-for-sale securities, accumulated gross unrealized gain (loss), before tax	(1)		(2)
Available for sale (AFS) fixed maturities	1,100		1,237
Other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available for sale, amortized cost	2,425		2,463
Fixed maturities, available for sale, allowance for expected credit losses	5		6
Fixed maturities, available for sale, gross unrealized, gains	27		19
Fixed maturities, available for sale, gross unrealized, losses	(44)		(69)
Fixed maturities, available-for-sale securities, accumulated gross unrealized gain (loss), before tax	(17)		(50)
Available for sale (AFS) fixed maturities	2,403		2,407
Corporate and other
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, available for sale, amortized cost	3,373		3,542
Fixed maturities, available for sale, allowance for expected credit losses	5	$ 5	23	$ 15	$ 0	$ 3
Fixed maturities, available for sale, gross unrealized, gains	86		42
Fixed maturities, available for sale, gross unrealized, losses	(26)		(65)
Fixed maturities, available-for-sale securities, accumulated gross unrealized gain (loss), before tax	60		(23)
Available for sale (AFS) fixed maturities	$ 3,428		$ 3,496

AMERICAN FINANCIAL GROUP INC 10-Q