Securities (Securities with Unrealized Losses) (Details) $ in Millions	Jun. 30, 2024 USD ($)	Dec. 31, 2023 USD ($) security
FAIR VALUE
LESS THAN 12 MONTHS	$ 3,254.9	$ 503.0
12 MONTHS OR LONGER	10,434.0	18,932.3
TOTAL	13,688.9	19,435.3
UNREALIZED LOSSES
LESS THAN 12 MONTHS	6.1	24.1
12 MONTHS OR LONGER	292.1	544.2
TOTAL	298.2	568.3
U.S. Government
FAIR VALUE
LESS THAN 12 MONTHS	792.1	0.0
12 MONTHS OR LONGER	719.9	3,364.7
TOTAL	1,512.0	3,364.7
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.2	0.0
12 MONTHS OR LONGER	18.3	61.5
TOTAL	18.5	61.5
Obligations of States and Political Subdivisions
FAIR VALUE
LESS THAN 12 MONTHS	0.0	87.8
12 MONTHS OR LONGER	293.4	208.0
TOTAL	293.4	295.8
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.0	5.9
12 MONTHS OR LONGER	15.3	14.1
TOTAL	15.3	20.0
Government Sponsored Agency
FAIR VALUE
LESS THAN 12 MONTHS	1,259.6	331.0
12 MONTHS OR LONGER	6,901.0	9,486.6
TOTAL	8,160.6	9,817.6
UNREALIZED LOSSES
LESS THAN 12 MONTHS	2.4	11.5
12 MONTHS OR LONGER	130.0	188.8
TOTAL	132.4	200.3
Non-U.S. Government
FAIR VALUE
LESS THAN 12 MONTHS	0.0	0.0
12 MONTHS OR LONGER	256.6	264.5
TOTAL	256.6	264.5
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.0	0.0
12 MONTHS OR LONGER	18.2	20.4
TOTAL	18.2	20.4
Corporate Debt
FAIR VALUE
LESS THAN 12 MONTHS	2.6	4.4
12 MONTHS OR LONGER	83.6	143.0
TOTAL	86.2	147.4
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.0	0.1
12 MONTHS OR LONGER	1.3	2.1
TOTAL	1.3	$ 2.2
Number of debt securities, available-for-sale | security		3
Debt securities, available-for-sale, continuous unrealized loss position with credit loss	81.8	$ 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	0.6	1.2
Covered Bonds
FAIR VALUE
LESS THAN 12 MONTHS	0.0	0.0
12 MONTHS OR LONGER	168.1	213.2
TOTAL	168.1	213.2
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.0	0.0
12 MONTHS OR LONGER	5.4	9.7
TOTAL	5.4	9.7
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
FAIR VALUE
LESS THAN 12 MONTHS	1,137.1	0.0
12 MONTHS OR LONGER	876.6	2,477.0
TOTAL	2,013.7	2,477.0
UNREALIZED LOSSES
LESS THAN 12 MONTHS	3.2	0.0
12 MONTHS OR LONGER	57.1	105.8
TOTAL	60.3	$ 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	62.5	19.8
12 MONTHS OR LONGER	538.8	1,998.7
TOTAL	601.3	2,018.5
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.3	2.0
12 MONTHS OR LONGER	26.0	98.0
TOTAL	26.3	100.0
Commercial Mortgage-Backed
FAIR VALUE
LESS THAN 12 MONTHS	1.0	60.0
12 MONTHS OR LONGER	596.0	776.6
TOTAL	597.0	836.6
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.0	4.6
12 MONTHS OR LONGER	20.5	43.8
TOTAL	$ 20.5	$ 48.4