Fair Value Measurements (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
Assets of AAMI and consolidated Funds
Total fair value assets	$ 141.6	$ 221.9
Liabilities of consolidated Funds
Total fair value liabilities	$ (9.3)	(20.7)
Real estate investment funds
Liabilities of consolidated Funds
Term over which the underlying assets are expected to be liquidated by the investees	1 year
Quoted prices in active markets (Level I)
Assets of AAMI and consolidated Funds
Total fair value assets	$ 61.5	142.7
Liabilities of consolidated Funds
Total fair value liabilities	(7.8)	(20.4)
Significant other observable inputs (Level II)
Assets of AAMI and consolidated Funds
Total fair value assets	66.8	59.8
Liabilities of consolidated Funds
Total fair value liabilities	(1.5)	(0.3)
Significant unobservable inputs (Level III)
Assets of AAMI and consolidated Funds
Total fair value assets	0.0	0.0
Liabilities of consolidated Funds
Total fair value liabilities	0.0	0.0
Uncategorized
Assets of AAMI and consolidated Funds
Total fair value assets	13.3	19.4
Consolidated Funds
Assets of AAMI and consolidated Funds
Common and preferred stock	23.6	94.2
Derivatives	1.8	0.7
Investments in unconsolidated Funds	90.4	154.0
Total fair value assets	90.4	154.0
Liabilities of consolidated Funds
Securities sold short	(7.8)	(20.4)
Derivatives	(1.5)	(0.3)
Total fair value liabilities	(9.3)	(20.7)
Consolidated Funds | Corporate bonds
Assets of AAMI and consolidated Funds
Corporate bonds	65.0	59.1
Consolidated Funds | Quoted prices in active markets (Level I)
Assets of AAMI and consolidated Funds
Common and preferred stock	23.6	94.2
Derivatives	0.0	0.0
Total fair value assets	23.6	94.2
Liabilities of consolidated Funds
Securities sold short	(7.8)	(20.4)
Derivatives	0.0	0.0
Total fair value liabilities	(7.8)	(20.4)
Consolidated Funds | Quoted prices in active markets (Level I) | Corporate bonds
Assets of AAMI and consolidated Funds
Corporate bonds	0.0	0.0
Consolidated Funds | Significant other observable inputs (Level II)
Assets of AAMI and consolidated Funds
Common and preferred stock	0.0	0.0
Derivatives	1.8	0.7
Total fair value assets	66.8	59.8
Liabilities of consolidated Funds
Securities sold short	0.0	0.0
Derivatives	(1.5)	(0.3)
Total fair value liabilities	(1.5)	(0.3)
Consolidated Funds | Significant other observable inputs (Level II) | Corporate bonds
Assets of AAMI and consolidated Funds
Corporate bonds	65.0	59.1
Consolidated Funds | Significant unobservable inputs (Level III)
Assets of AAMI and consolidated Funds
Common and preferred stock	0.0	0.0
Derivatives	0.0	0.0
Total fair value assets	0.0	0.0
Liabilities of consolidated Funds
Securities sold short	0.0	0.0
Derivatives	0.0	0.0
Total fair value liabilities	0.0	0.0
Consolidated Funds | Significant unobservable inputs (Level III) | Corporate bonds
Assets of AAMI and consolidated Funds
Corporate bonds	0.0	0.0
Consolidated Entity Excluding Consolidated Funds
Assets of AAMI and consolidated Funds
Investments in unconsolidated Funds	13.3	19.4
Total fair value assets	51.2	67.9
Consolidated Entity Excluding Consolidated Funds | Investments related to long-term incentive compensation plans
Assets of AAMI and consolidated Funds
Investments related to long-term incentive compensation plans	37.9	48.5
Consolidated Entity Excluding Consolidated Funds | Investment funds subject to longer redemption restrictions
Assets of AAMI and consolidated Funds
Investments in unconsolidated Funds	3.1	2.9
Consolidated Entity Excluding Consolidated Funds | Quoted prices in active markets (Level I)
Assets of AAMI and consolidated Funds
Investments in unconsolidated Funds	0.0	0.0
Total fair value assets	37.9	48.5
Consolidated Entity Excluding Consolidated Funds | Quoted prices in active markets (Level I) | Investments related to long-term incentive compensation plans
Assets of AAMI and consolidated Funds
Investments related to long-term incentive compensation plans	37.9	48.5
Consolidated Entity Excluding Consolidated Funds | Significant other observable inputs (Level II)
Assets of AAMI and consolidated Funds
Investments in unconsolidated Funds	0.0	0.0
Total fair value assets	0.0	0.0
Consolidated Entity Excluding Consolidated Funds | Significant other observable inputs (Level II) | Investments related to long-term incentive compensation plans
Assets of AAMI and consolidated Funds
Investments related to long-term incentive compensation plans	0.0	0.0
Consolidated Entity Excluding Consolidated Funds | Significant unobservable inputs (Level III)
Assets of AAMI and consolidated Funds
Investments in unconsolidated Funds	0.0	0.0
Total fair value assets	0.0	0.0
Consolidated Entity Excluding Consolidated Funds | Significant unobservable inputs (Level III) | Investments related to long-term incentive compensation plans
Assets of AAMI and consolidated Funds
Investments related to long-term incentive compensation plans	0.0	0.0
Consolidated Entity Excluding Consolidated Funds | Uncategorized
Assets of AAMI and consolidated Funds
Investments in unconsolidated Funds	13.3	19.4
Total fair value assets	$ 13.3	$ 19.4