Securities (Securities with Unrealized Losses) (Details) $ in Millions	Mar. 31, 2024 USD ($)	Dec. 31, 2023 USD ($) Security
FAIR VALUE
LESS THAN 12 MONTHS	$ 818.4	$ 503.0
12 MONTHS OR LONGER	12,703.2	18,932.3
TOTAL	13,521.6	19,435.3
UNREALIZED LOSSES
LESS THAN 12 MONTHS	43.3	24.1
12 MONTHS OR LONGER	284.7	544.2
TOTAL	328.0	568.3
U.S. Government
FAIR VALUE
LESS THAN 12 MONTHS	87.4	0.0
12 MONTHS OR LONGER	819.5	3,364.7
TOTAL	906.9	3,364.7
UNREALIZED LOSSES
LESS THAN 12 MONTHS	12.0	0.0
12 MONTHS OR LONGER	8.3	61.5
TOTAL	20.3	61.5
Obligations of States and Political Subdivisions
FAIR VALUE
LESS THAN 12 MONTHS	295.0	87.8
12 MONTHS OR LONGER	0.0	208.0
TOTAL	295.0	295.8
UNREALIZED LOSSES
LESS THAN 12 MONTHS	14.6	5.9
12 MONTHS OR LONGER	0.0	14.1
TOTAL	14.6	20.0
Government Sponsored Agency
FAIR VALUE
LESS THAN 12 MONTHS	145.3	331.0
12 MONTHS OR LONGER	8,865.2	9,486.6
TOTAL	9,010.5	9,817.6
UNREALIZED LOSSES
LESS THAN 12 MONTHS	7.9	11.5
12 MONTHS OR LONGER	155.1	188.8
TOTAL	163.0	200.3
Non-U.S. Government
FAIR VALUE
LESS THAN 12 MONTHS	63.5	0.0
12 MONTHS OR LONGER	189.5	264.5
TOTAL	253.0	264.5
UNREALIZED LOSSES
LESS THAN 12 MONTHS	1.3	0.0
12 MONTHS OR LONGER	16.2	20.4
TOTAL	17.5	20.4
Corporate Debt
FAIR VALUE
LESS THAN 12 MONTHS	36.9	4.4
12 MONTHS OR LONGER	55.7	143.0
TOTAL	92.6	147.4
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.9	0.1
12 MONTHS OR LONGER	0.9	2.1
TOTAL	1.8	$ 2.2
Number of debt securities, available-for-sale | Security		3
Debt securities, available-for-sale, continuous unrealized loss position with credit loss	80.1	$ 98.4
Debt securities, available-for-sale, continuous unrealized loss position, accumulated loss, with credit loss	4.7	5.9
Net amount of recognized liabilities for repurchase agreements	0.5	1.2
Covered Bonds
FAIR VALUE
LESS THAN 12 MONTHS	60.4	0.0
12 MONTHS OR LONGER	104.9	213.2
TOTAL	165.3	213.2
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.5	0.0
12 MONTHS OR LONGER	5.7	9.7
TOTAL	6.2	9.7
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
FAIR VALUE
LESS THAN 12 MONTHS	110.0	0.0
12 MONTHS OR LONGER	1,066.0	2,477.0
TOTAL	1,176.0	2,477.0
UNREALIZED LOSSES
LESS THAN 12 MONTHS	5.0	0.0
12 MONTHS OR LONGER	50.6	105.8
TOTAL	55.6	$ 105.8
Number of debt securities, available-for-sale | Security		1
Debt securities, available-for-sale, continuous unrealized loss position with credit loss		$ 71.0
Debt securities, available-for-sale, continuous unrealized loss position, accumulated loss, with credit loss		8.2
Other Asset-Backed
FAIR VALUE
LESS THAN 12 MONTHS	13.9	19.8
12 MONTHS OR LONGER	995.0	1,998.7
TOTAL	1,008.9	2,018.5
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.7	2.0
12 MONTHS OR LONGER	30.3	98.0
TOTAL	31.0	100.0
Commercial Mortgage-Backed
FAIR VALUE
LESS THAN 12 MONTHS	6.0	60.0
12 MONTHS OR LONGER	607.4	776.6
TOTAL	613.4	836.6
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.4	4.6
12 MONTHS OR LONGER	17.6	43.8
TOTAL	$ 18.0	$ 48.4