Investments - Schedule of Changes in Other Investments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Other Investments [Roll Forward]
Balance, beginning of period	$ 566.1	$ 530.3	$ 539.0	$ 480.9
Purchases and commitments funded	19.6	13.6	68.5	78.1
Sales and distributions	(25.4)	(20.4)	(67.3)	(47.1)
Net realized and unrealized gains	13.9	9.3	34.0	20.9
Balance, end of period	574.2	532.8	574.2	532.8
Equity Securities Without Readily Determinable Fair Value [Roll Forward]
Balance, beginning of period	50.4	50.4	50.4	50.4
Purchases and commitments funded	0.0	0.0	0.0	0.0
Sales and distributions	0.0	0.0	0.0	0.0
Net realized and unrealized gains	0.0	0.0	0.0	0.0
Balance, end of period	50.4	50.4	50.4	50.4
Fair Value Measured at NAV
Other Investments [Roll Forward]
Balance, beginning of period	515.7	479.9	488.6	430.5
Purchases and commitments funded	19.6	13.6	68.5	78.1
Sales and distributions	(25.4)	(20.4)	(67.3)	(47.1)
Net realized and unrealized gains	13.9	9.3	34.0	20.9
Balance, end of period	$ 523.8	$ 482.4	$ 523.8	$ 482.4

AFFILIATED MANAGERS GROUP, INC. 10-Q