Securities (Securities with Unrealized Losses) (Details) $ in Millions	Mar. 31, 2023 USD ($) Security	Dec. 31, 2022 USD ($) Security	Mar. 31, 2022 USD ($)
FAIR VALUE
LESS THAN 12 MONTHS	$ 12,639.1	$ 15,665.4
12 MONTHS OR LONGER	8,248.0	5,769.4
TOTAL	20,887.1	21,434.8	$ 20,900.0
UNREALIZED LOSSES
LESS THAN 12 MONTHS	447.4	727.7
12 MONTHS OR LONGER	359.6	321.5
TOTAL	807.0	1,049.2
U.S. Government
FAIR VALUE
LESS THAN 12 MONTHS	949.8	1,123.6
12 MONTHS OR LONGER	534.3	343.1
TOTAL	1,484.1	1,466.7
UNREALIZED LOSSES
LESS THAN 12 MONTHS	39.5	64.1
12 MONTHS OR LONGER	36.5	28.7
TOTAL	76.0	92.8
Obligations of States and Political Subdivisions
FAIR VALUE
LESS THAN 12 MONTHS	18.5	160.0
12 MONTHS OR LONGER	190.7	120.6
TOTAL	209.2	280.6
UNREALIZED LOSSES
LESS THAN 12 MONTHS	1.8	16.8
12 MONTHS OR LONGER	17.6	13.4
TOTAL	$ 19.4	30.2
Number of debt securities, available-for-sale | Security	3
Debt securities, available-for-sale, continuous unrealized loss position with credit loss	$ 82.7
Debt securities, available-for-sale, continuous unrealized loss position, accumulated loss, with credit loss	6.8
Government Sponsored Agency
FAIR VALUE
LESS THAN 12 MONTHS	7,010.8	7,631.4
12 MONTHS OR LONGER	3,490.2	2,737.7
TOTAL	10,501.0	10,369.1
UNREALIZED LOSSES
LESS THAN 12 MONTHS	181.7	262.1
12 MONTHS OR LONGER	76.8	89.5
TOTAL	258.5	351.6
Non-U.S. Government
FAIR VALUE
LESS THAN 12 MONTHS	66.6	235.4
12 MONTHS OR LONGER	294.9	124.6
TOTAL	361.5	360.0
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.0	17.3
12 MONTHS OR LONGER	20.6	10.3
TOTAL	20.6	27.6
Corporate Debt
FAIR VALUE
LESS THAN 12 MONTHS	196.3	427.3
12 MONTHS OR LONGER	172.3	130.3
TOTAL	368.6	557.6
UNREALIZED LOSSES
LESS THAN 12 MONTHS	8.4	14.6
12 MONTHS OR LONGER	0.8	2.8
TOTAL	$ 9.2	$ 17.4
Number of debt securities, available-for-sale | Security	6	3
Debt securities, available-for-sale, continuous unrealized loss position with credit loss	$ 431.3	$ 93.8
Debt securities, available-for-sale, continuous unrealized loss position, accumulated loss, with credit loss	26.7	9.5
Net amount of recognized liabilities for repurchase agreements	1.8	1.3
Covered Bonds
FAIR VALUE
LESS THAN 12 MONTHS	131.4	238.0
12 MONTHS OR LONGER	149.4	42.8
TOTAL	280.8	280.8
UNREALIZED LOSSES
LESS THAN 12 MONTHS	9.3	13.5
12 MONTHS OR LONGER	2.6	1.2
TOTAL	11.9	14.7
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
FAIR VALUE
LESS THAN 12 MONTHS	1,070.1	1,305.4
12 MONTHS OR LONGER	826.3	807.1
TOTAL	1,896.4	2,112.5
UNREALIZED LOSSES
LESS THAN 12 MONTHS	38.9	74.4
12 MONTHS OR LONGER	67.5	83.2
TOTAL	$ 106.4	$ 157.6
Number of debt securities, available-for-sale | Security	11	1
Debt securities, available-for-sale, continuous unrealized loss position with credit loss	$ 148.7	$ 68.3
Debt securities, available-for-sale, continuous unrealized loss position, accumulated loss, with credit loss	9.2	9.1
Other Asset-Backed
FAIR VALUE
LESS THAN 12 MONTHS	2,672.5	3,873.4
12 MONTHS OR LONGER	2,226.3	1,247.6
TOTAL	4,898.8	5,121.0
UNREALIZED LOSSES
LESS THAN 12 MONTHS	130.6	217.5
12 MONTHS OR LONGER	107.1	70.6
TOTAL	237.7	288.1
Commercial Mortgage-Backed
FAIR VALUE
LESS THAN 12 MONTHS	523.1	670.9
12 MONTHS OR LONGER	363.6	215.6
TOTAL	886.7	886.5
UNREALIZED LOSSES
LESS THAN 12 MONTHS	37.2	47.4
12 MONTHS OR LONGER	30.1	21.8
TOTAL	$ 67.3	$ 69.2