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		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	$ 16.7	$ 15.4	$ 16.0	$ 11.2
Provision for (Release of) Credit Losses	(1.8)	(0.4)	(1.1)	3.8
Balance at End of Period	14.9	15.0	14.9	15.0
CORPORATE DEBT
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	1.4	1.8	1.9	1.4
Provision for (Release of) Credit Losses	(0.1)	(0.3)	(0.6)	0.1
Balance at End of Period	1.3	1.5	1.3	1.5
NON-U.S. GOVERNMENT
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	5.4	3.8	3.6	1.9
Provision for (Release of) Credit Losses	(0.8)	(0.7)	1.0	1.2
Balance at End of Period	4.6	3.1	4.6	3.1
SUB-SOVEREIGN, SUPRANATIONAL, AND NON-U.S. AGENCY BONDS
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	3.4	4.8	4.0	3.0
Provision for (Release of) Credit Losses	(0.5)	(1.0)	(1.1)	0.8
Balance at End of Period	2.9	3.8	2.9	3.8
OBLIGATIONS OF STATES AND POICITAL SUBDIVISIONS
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	1.3	0.0	1.5	0.0
Provision for (Release of) Credit Losses	(0.1)	0.0	(0.3)	0.0
Balance at End of Period	1.2	0.0	1.2	0.0
COVERED BONDS
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	0.1	0.1	0.1	0.1
Provision for (Release of) Credit Losses	0.0	0.0	0.0	0.0
Balance at End of Period	0.1	0.1	0.1	0.1
OTHER
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	5.1	4.9	4.9	4.8
Provision for (Release of) Credit Losses	(0.3)	1.6	(0.1)	1.7
Balance at End of Period	$ 4.8	$ 6.5	$ 4.8	$ 6.5