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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	$ 10.5	$ 6.5	$ 7.3	$ 0.0
Provision for Credit Losses	(0.1)	0.4	3.1	0.3
Balance at End of Period	10.4	6.9	10.4	6.9
Cumulative Effect, Period of Adoption, Adjustment
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period				6.6
Adjusted Balance
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period				6.6
CORPORATE DEBT
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	1.6	0.5	0.8	0.0
Provision for Credit Losses	(0.2)	0.2	0.6	(0.1)
Balance at End of Period	1.4	0.7	1.4	0.7
CORPORATE DEBT | Cumulative Effect, Period of Adoption, Adjustment
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period				0.8
CORPORATE DEBT | Adjusted Balance
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period				0.8
NON-U.S. GOVERNMENT
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	1.5	0.4	0.2	0.0
Provision for Credit Losses	(0.1)	0.0	1.2	0.1
Balance at End of Period	1.4	0.4	1.4	0.4
NON-U.S. GOVERNMENT | Cumulative Effect, Period of Adoption, Adjustment
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period				0.3
NON-U.S. GOVERNMENT | Adjusted Balance
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period				0.3
SUB-SOVEREIGN, SUPRANATIONAL, AND NON-U.S. AGENCY BONDS
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	2.6	0.7	1.2	0.0
Provision for Credit Losses	0.2	(0.1)	1.6	(0.3)
Balance at End of Period	2.8	0.6	2.8	0.6
SUB-SOVEREIGN, SUPRANATIONAL, AND NON-U.S. AGENCY BONDS | Cumulative Effect, Period of Adoption, Adjustment
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period				0.9
SUB-SOVEREIGN, SUPRANATIONAL, AND NON-U.S. AGENCY BONDS | Adjusted Balance
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period				0.9
CERTIFICATE OF DEPOSITS
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	0.0	0.0	0.0	0.0
Provision for Credit Losses	0.0	0.2	0.0	0.2
Balance at End of Period	0.0	0.2	0.0	0.2
CERTIFICATE OF DEPOSITS | Cumulative Effect, Period of Adoption, Adjustment
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period				0.0
CERTIFICATE OF DEPOSITS | Adjusted Balance
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period				0.0
COVERED BONDS
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	0.1	0.0	0.1	0.0
Provision for Credit Losses	0.0	0.1	0.0	0.1
Balance at End of Period	0.1	0.1	0.1	0.1
COVERED BONDS | Cumulative Effect, Period of Adoption, Adjustment
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period				0.0
COVERED BONDS | Adjusted Balance
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period				0.0
OTHER
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period	4.7	4.9	5.0	0.0
Provision for Credit Losses	0.0	0.0	(0.3)	0.3
Balance at End of Period	$ 4.7	$ 4.9	$ 4.7	4.9
OTHER | Cumulative Effect, Period of Adoption, Adjustment
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period				4.6
OTHER | Adjusted Balance
HELD TO MATURITY DEBT SECURITIES
Balance at Beginning of Period				$ 4.6