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, 2020	Mar. 31, 2019
DEBT SECURITIES HELD TO MATURITY
Balance at beginning of period	$ 0.0
Charge-Offs	0.0	$ 0.0
Recoveries	0.0	0.0
Net (Charge-Offs) Recoveries	0.0	0.0
Provision for Credit Losses	2.9	0.0
Balance at end of period	9.5
Cumulative Effect Adjustment
DEBT SECURITIES HELD TO MATURITY
Balance at beginning of period	6.6
Adjusted Balance
DEBT SECURITIES HELD TO MATURITY
Balance at beginning of period	6.6	0.0
Balance at end of period		$ 0.0
CORPORATE DEBT
DEBT SECURITIES HELD TO MATURITY
Balance at beginning of period	0.0
Charge-Offs	0.0
Recoveries	0.0
Net (Charge-Offs) Recoveries	0.0
Provision for Credit Losses	1.9
Balance at end of period	2.7
CORPORATE DEBT | Cumulative Effect Adjustment
DEBT SECURITIES HELD TO MATURITY
Balance at beginning of period	0.8
CORPORATE DEBT | Adjusted Balance
DEBT SECURITIES HELD TO MATURITY
Balance at beginning of period	0.8
NON-U.S. GOVERNMENT
DEBT SECURITIES HELD TO MATURITY
Balance at beginning of period	0.0
Charge-Offs	0.0
Recoveries	0.0
Net (Charge-Offs) Recoveries	0.0
Provision for Credit Losses	0.1
Balance at end of period	0.4
NON-U.S. GOVERNMENT | Cumulative Effect Adjustment
DEBT SECURITIES HELD TO MATURITY
Balance at beginning of period	0.3
NON-U.S. GOVERNMENT | Adjusted Balance
DEBT SECURITIES HELD TO MATURITY
Balance at beginning of period	0.3
SUB-SOVEREIGN, SUPERNATIONAL, AND NON-U.S. AGENCY BONDS
DEBT SECURITIES HELD TO MATURITY
Balance at beginning of period	0.0
Charge-Offs	0.0
Recoveries	0.0
Net (Charge-Offs) Recoveries	0.0
Provision for Credit Losses	0.4
Balance at end of period	1.3
SUB-SOVEREIGN, SUPERNATIONAL, AND NON-U.S. AGENCY BONDS | Cumulative Effect Adjustment
DEBT SECURITIES HELD TO MATURITY
Balance at beginning of period	0.9
SUB-SOVEREIGN, SUPERNATIONAL, AND NON-U.S. AGENCY BONDS | Adjusted Balance
DEBT SECURITIES HELD TO MATURITY
Balance at beginning of period	0.9
CERTIFICATE OF DEPOSITS
DEBT SECURITIES HELD TO MATURITY
Balance at beginning of period	0.0
Charge-Offs	0.0
Recoveries	0.0
Net (Charge-Offs) Recoveries	0.0
Provision for Credit Losses	0.2
Balance at end of period	0.2
CERTIFICATE OF DEPOSITS | Cumulative Effect Adjustment
DEBT SECURITIES HELD TO MATURITY
Balance at beginning of period	0.0
CERTIFICATE OF DEPOSITS | Adjusted Balance
DEBT SECURITIES HELD TO MATURITY
Balance at beginning of period	0.0
COVERED BONDS
DEBT SECURITIES HELD TO MATURITY
Balance at beginning of period	0.0
Charge-Offs	0.0
Recoveries	0.0
Net (Charge-Offs) Recoveries	0.0
Provision for Credit Losses	0.1
Balance at end of period	0.1
COVERED BONDS | Cumulative Effect Adjustment
DEBT SECURITIES HELD TO MATURITY
Balance at beginning of period	0.0
COVERED BONDS | Adjusted Balance
DEBT SECURITIES HELD TO MATURITY
Balance at beginning of period	0.0
OTHER
DEBT SECURITIES HELD TO MATURITY
Balance at beginning of period	0.0
Charge-Offs	0.0
Recoveries	0.0
Net (Charge-Offs) Recoveries	0.0
Provision for Credit Losses	0.2
Balance at end of period	4.8
OTHER | Cumulative Effect Adjustment
DEBT SECURITIES HELD TO MATURITY
Balance at beginning of period	4.6
OTHER | Adjusted Balance
DEBT SECURITIES HELD TO MATURITY
Balance at beginning of period	$ 4.6