Securities (Securities with Unrealized Losses) (Details) $ in Millions	Jun. 30, 2022 USD ($) Security	Dec. 31, 2021 USD ($)
FAIR VALUE
LESS THAN 12 MONTHS	$ 24,427.4	$ 16,660.7
12 MONTHS OR LONGER	6,226.1	2,037.0
TOTAL	30,653.5	18,697.7
UNREALIZED LOSSES
LESS THAN 12 MONTHS	1,096.6	223.6
12 MONTHS OR LONGER	739.1	25.0
TOTAL	1,835.7	248.6
U.S. Government
FAIR VALUE
LESS THAN 12 MONTHS	988.2	239.0
12 MONTHS OR LONGER	219.1	0.0
TOTAL	1,207.3	239.0
UNREALIZED LOSSES
LESS THAN 12 MONTHS	31.7	9.4
12 MONTHS OR LONGER	29.5	0.0
TOTAL	61.2	9.4
Obligations of States and Political Subdivisions
FAIR VALUE
LESS THAN 12 MONTHS	2,492.4	1,699.5
12 MONTHS OR LONGER	871.9	31.6
TOTAL	3,364.3	1,731.1
UNREALIZED LOSSES
LESS THAN 12 MONTHS	261.3	37.4
12 MONTHS OR LONGER	168.7	1.2
TOTAL	430.0	38.6
Government Sponsored Agency
FAIR VALUE
LESS THAN 12 MONTHS	11,012.3	8,207.3
12 MONTHS OR LONGER	3,833.4	1,821.4
TOTAL	14,845.7	10,028.7
UNREALIZED LOSSES
LESS THAN 12 MONTHS	348.4	90.2
12 MONTHS OR LONGER	403.2	20.0
TOTAL	751.6	110.2
Non-U.S. Government
FAIR VALUE
LESS THAN 12 MONTHS	168.4	230.0
12 MONTHS OR LONGER	198.9	0.0
TOTAL	367.3	230.0
UNREALIZED LOSSES
LESS THAN 12 MONTHS	3.4	9.5
12 MONTHS OR LONGER	25.5	0.0
TOTAL	28.9	9.5
Corporate Debt
FAIR VALUE
LESS THAN 12 MONTHS	1,653.9	693.7
12 MONTHS OR LONGER	53.3	0.0
TOTAL	1,707.2	693.7
UNREALIZED LOSSES
LESS THAN 12 MONTHS	57.9	9.7
12 MONTHS OR LONGER	5.6	0.0
TOTAL	$ 63.5	9.7
Number of debt securities, available-for-sale | Security	5
Debt securities, available-for-sale, continuous unrealized loss position with credit loss	$ 144.4
Debt securities, available-for-sale, continuous unrealized loss position, accumulated loss, with credit loss	12.8
Covered Bonds
FAIR VALUE
LESS THAN 12 MONTHS	407.2	92.1
12 MONTHS OR LONGER	0.0	0.0
TOTAL	407.2	92.1
UNREALIZED LOSSES
LESS THAN 12 MONTHS	12.9	0.9
12 MONTHS OR LONGER	0.0	0.0
TOTAL	12.9	0.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
FAIR VALUE
LESS THAN 12 MONTHS	1,238.1	1,116.8
12 MONTHS OR LONGER	731.4	172.3
TOTAL	1,969.5	1,289.1
UNREALIZED LOSSES
LESS THAN 12 MONTHS	58.7	31.9
12 MONTHS OR LONGER	82.1	3.5
TOTAL	$ 140.8	35.4
Number of debt securities, available-for-sale | Security	1
Debt securities, available-for-sale, continuous unrealized loss position with credit loss	$ 70.7
Debt securities, available-for-sale, continuous unrealized loss position, accumulated loss, with credit loss	6.7
Other Asset-Backed
FAIR VALUE
LESS THAN 12 MONTHS	5,120.4	3,815.5
12 MONTHS OR LONGER	271.1	11.7
TOTAL	5,391.5	3,827.2
UNREALIZED LOSSES
LESS THAN 12 MONTHS	253.0	31.4
12 MONTHS OR LONGER	18.7	0.3
TOTAL	271.7	31.7
Commercial Mortgage-Backed
FAIR VALUE
LESS THAN 12 MONTHS	1,346.5	566.8
12 MONTHS OR LONGER	47.0	0.0
TOTAL	1,393.5	566.8
UNREALIZED LOSSES
LESS THAN 12 MONTHS	69.3	3.2
12 MONTHS OR LONGER	5.8	0.0
TOTAL	$ 75.1	$ 3.2