Securities (Securities with Unrealized Losses) (Details) $ in Millions	Mar. 31, 2022 USD ($) Security	Dec. 31, 2021 USD ($)
FAIR VALUE
LESS THAN 12 MONTHS	$ 23,516.1	$ 16,660.7
12 MONTHS OR LONGER	2,437.2	2,037.0
TOTAL	25,953.3	18,697.7
Debt securities, available-for-sale, continuous unrealized loss position with credit loss	80.7
UNREALIZED LOSSES
LESS THAN 12 MONTHS	1,033.4	223.6
12 MONTHS OR LONGER	124.3	25.0
TOTAL	$ 1,157.7	248.6
Number of debt securities, available-for-sale | Security	3
Debt securities, available-for-sale, continuous unrealized loss position, accumulated loss, with credit loss	$ 6.7
U.S. Government
FAIR VALUE
LESS THAN 12 MONTHS	956.2	239.0
12 MONTHS OR LONGER	0.0	0.0
TOTAL	956.2	239.0
UNREALIZED LOSSES
LESS THAN 12 MONTHS	41.3	9.4
12 MONTHS OR LONGER	0.0	0.0
TOTAL	41.3	9.4
Obligations of States and Political Subdivisions
FAIR VALUE
LESS THAN 12 MONTHS	3,303.5	1,699.5
12 MONTHS OR LONGER	46.8	31.6
TOTAL	3,350.3	1,731.1
UNREALIZED LOSSES
LESS THAN 12 MONTHS	250.5	37.4
12 MONTHS OR LONGER	6.1	1.2
TOTAL	256.6	38.6
Government Sponsored Agency
FAIR VALUE
LESS THAN 12 MONTHS	9,106.8	8,207.3
12 MONTHS OR LONGER	2,103.3	1,821.4
TOTAL	11,210.1	10,028.7
UNREALIZED LOSSES
LESS THAN 12 MONTHS	379.8	90.2
12 MONTHS OR LONGER	96.1	20.0
TOTAL	475.9	110.2
Non-U.S. Government
FAIR VALUE
LESS THAN 12 MONTHS	312.4	230.0
12 MONTHS OR LONGER	75.5	0.0
TOTAL	387.9	230.0
UNREALIZED LOSSES
LESS THAN 12 MONTHS	15.3	9.5
12 MONTHS OR LONGER	9.1	0.0
TOTAL	24.4	9.5
Corporate Debt
FAIR VALUE
LESS THAN 12 MONTHS	1,357.3	693.7
12 MONTHS OR LONGER	0.0	0.0
TOTAL	1,357.3	693.7
UNREALIZED LOSSES
LESS THAN 12 MONTHS	45.2	9.7
12 MONTHS OR LONGER	0.0	0.0
TOTAL	45.2	9.7
Covered Bonds
FAIR VALUE
LESS THAN 12 MONTHS	242.1	92.1
12 MONTHS OR LONGER	0.0	0.0
TOTAL	242.1	92.1
UNREALIZED LOSSES
LESS THAN 12 MONTHS	7.9	0.9
12 MONTHS OR LONGER	0.0	0.0
TOTAL	7.9	0.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
FAIR VALUE
LESS THAN 12 MONTHS	1,667.1	1,116.8
12 MONTHS OR LONGER	200.3	172.3
TOTAL	1,867.4	1,289.1
UNREALIZED LOSSES
LESS THAN 12 MONTHS	101.7	31.9
12 MONTHS OR LONGER	12.3	3.5
TOTAL	114.0	35.4
Other Asset-Backed
FAIR VALUE
LESS THAN 12 MONTHS	5,441.0	3,815.5
12 MONTHS OR LONGER	11.3	11.7
TOTAL	5,452.3	3,827.2
UNREALIZED LOSSES
LESS THAN 12 MONTHS	156.8	31.4
12 MONTHS OR LONGER	0.7	0.3
TOTAL	157.5	31.7
Commercial Mortgage-Backed
FAIR VALUE
LESS THAN 12 MONTHS	1,129.7	566.8
12 MONTHS OR LONGER	0.0	0.0
TOTAL	1,129.7	566.8
UNREALIZED LOSSES
LESS THAN 12 MONTHS	34.9	3.2
12 MONTHS OR LONGER	0.0	0.0
TOTAL	$ 34.9	$ 3.2