Allowance for Credit Losses (Changes in The Allowance for Credit Losses Related to Debt Securities Held to Maturity) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	$ 12.7	$ 16.0
Provision for Credit Losses	(0.3)	0.0
Balance at End of Period	12.4	16.0
CORPORATE DEBT
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	0.9	1.9
Provision for Credit Losses	(0.1)	0.0
Balance at End of Period	0.8	1.9
NON-U.S. GOVERNMENT
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	3.5	3.6
Provision for Credit Losses	0.0	0.3
Balance at End of Period	3.5	3.9
SUB-SOVEREIGN, SUPRANATIONAL, AND NON-U.S. AGENCY BONDS
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	2.2	4.0
Provision for Credit Losses	(0.1)	0.1
Balance at End of Period	2.1	4.1
OBLIGATIONS OF STATES AND POICITAL SUBDIVISIONS
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	1.2	1.5
Provision for Credit Losses	(0.1)	(0.2)
Balance at End of Period	1.1	1.3
COVERED BONDS
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	0.1	0.1
Provision for Credit Losses	0.0	0.0
Balance at End of Period	0.1	0.1
OTHER
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	4.8	4.9
Provision for Credit Losses	0.0	(0.2)
Balance at End of Period	$ 4.8	$ 4.7