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, 2023	Dec. 31, 2022	Dec. 31, 2021
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	$ 16.0	$ 11.2	$ 7.3
Charge-Offs	(1.2)	0.0	0.0
Recoveries	0.0	0.0	0.0
Net Recoveries (Charge-Offs)	(1.2)
Provision for (Release of) Credit Losses	(2.1)	4.8	3.9
Balance at End of Period	12.7	16.0	11.2
CORPORATE DEBT
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	1.9	1.4	0.8
Charge-Offs	0.0
Recoveries	0.0
Net Recoveries (Charge-Offs)	0.0
Provision for (Release of) Credit Losses	(1.0)	0.5	0.6
Balance at End of Period	0.9	1.9	1.4
Foreign Corporate Bonds
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	3.6	1.9	0.2
Charge-Offs	0.0
Recoveries	0.0
Net Recoveries (Charge-Offs)	0.0
Provision for (Release of) Credit Losses	(0.1)	1.7	1.7
Balance at End of Period	3.5	3.6	1.9
SUB-SOVEREIGN, SUPRANATIONAL, AND NON-U.S. AGENCY BONDS
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	4.0	3.0	1.2
Charge-Offs	0.0
Recoveries	0.0
Net Recoveries (Charge-Offs)	0.0
Provision for (Release of) Credit Losses	(1.8)	1.0	1.8
Balance at End of Period	2.2	4.0	3.0
Obligations of States and Political Subdivisions
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	1.5	0.0	0.0
Charge-Offs	0.0
Recoveries	0.0
Net Recoveries (Charge-Offs)	0.0
Provision for (Release of) Credit Losses	(0.3)	1.5	0.0
Balance at End of Period	1.2	1.5	0.0
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 (Release of) Credit Losses	0.0	0.0	0.0
Balance at End of Period	0.1	0.1	0.1
OTHER
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	4.9	4.8	5.0
Charge-Offs	(1.2)
Recoveries	0.0
Net Recoveries (Charge-Offs)	(1.2)
Provision for (Release of) Credit Losses	1.1	0.1	(0.2)
Balance at End of Period	$ 4.8	$ 4.9	$ 4.8