Allowance for Credit Losses (Changes in The Allowance for Credit Losses Related to Debt Securities Held to Maturity) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	$ 12.7	$ 16.0	$ 11.2
Charge-Offs	0.0	(1.2)	0.0
Recoveries	0.0	0.0	0.0
Net Recoveries (Charge-Offs)		(1.2)
Provision for Credit Losses	(6.2)	(2.1)	4.8
Balance at End of Period	6.5	12.7	16.0
CORPORATE DEBT
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	0.9	1.9	1.4
Charge-Offs		0.0
Recoveries		0.0
Net Recoveries (Charge-Offs)		0.0
Provision for Credit Losses	(0.6)	(1.0)	0.5
Balance at End of Period	0.3	0.9	1.9
NON-U.S. GOVERNMENT
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	3.5	3.6	1.9
Charge-Offs		0.0
Recoveries		0.0
Net Recoveries (Charge-Offs)		0.0
Provision for Credit Losses	(1.5)	(0.1)	1.7
Balance at End of Period	2.0	3.5	3.6
SUB-SOVEREIGN, SUPRANATIONAL, AND NON-U.S. AGENCY BONDS
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	2.2	4.0	3.0
Charge-Offs		0.0
Recoveries		0.0
Net Recoveries (Charge-Offs)		0.0
Provision for Credit Losses	(1.1)	(1.8)	1.0
Balance at End of Period	1.1	2.2	4.0
OBLIGATIONS OF STATES AND POLITICAL SUBDIVISIONS
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	1.2	1.5	0.0
Charge-Offs		0.0
Recoveries		0.0
Net Recoveries (Charge-Offs)		0.0
Provision for Credit Losses	(0.3)	(0.3)	1.5
Balance at End of Period	0.9	1.2	1.5
COVERED BONDS
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	0.1	0.1	0.1
Charge-Offs		0.0
Recoveries		0.0
Net Recoveries (Charge-Offs)		0.0
Provision for Credit Losses	(0.1)	0.0	0.0
Balance at End of Period	0.0	0.1	0.1
OTHER
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	4.8	4.9	4.8
Charge-Offs		(1.2)
Recoveries		0.0
Net Recoveries (Charge-Offs)		(1.2)
Provision for Credit Losses	(2.6)	1.1	0.1
Balance at End of Period	$ 2.2	$ 4.8	$ 4.9