Securities (Securities with Unrealized Losses) (Details) $ in Millions	Sep. 30, 2024 USD ($)	Dec. 31, 2023 USD ($) security
FAIR VALUE
LESS THAN 12 MONTHS	$ 8,034.3	$ 503.0
12 MONTHS OR LONGER	10,098.5	18,932.3
TOTAL	18,132.8	19,435.3
UNREALIZED LOSSES
LESS THAN 12 MONTHS	11.2	24.1
12 MONTHS OR LONGER	232.7	544.2
TOTAL	243.9	568.3
U.S. Government
FAIR VALUE
LESS THAN 12 MONTHS	5,293.0	0.0
12 MONTHS OR LONGER	630.9	3,364.7
TOTAL	5,923.9	3,364.7
UNREALIZED LOSSES
LESS THAN 12 MONTHS	6.9	0.0
12 MONTHS OR LONGER	12.2	61.5
TOTAL	19.1	61.5
Obligations of States and Political Subdivisions
FAIR VALUE
LESS THAN 12 MONTHS	0.0	87.8
12 MONTHS OR LONGER	305.9	208.0
TOTAL	305.9	295.8
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.0	5.9
12 MONTHS OR LONGER	15.6	14.1
TOTAL	15.6	20.0
Government Sponsored Agency
FAIR VALUE
LESS THAN 12 MONTHS	2,028.2	331.0
12 MONTHS OR LONGER	6,676.9	9,486.6
TOTAL	8,705.1	9,817.6
UNREALIZED LOSSES
LESS THAN 12 MONTHS	3.6	11.5
12 MONTHS OR LONGER	110.7	188.8
TOTAL	114.3	200.3
Non-U.S. Government
FAIR VALUE
LESS THAN 12 MONTHS	55.3	0.0
12 MONTHS OR LONGER	271.9	264.5
TOTAL	327.2	264.5
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.0	0.0
12 MONTHS OR LONGER	14.4	20.4
TOTAL	14.4	20.4
Corporate Debt
FAIR VALUE
LESS THAN 12 MONTHS	0.0	4.4
12 MONTHS OR LONGER	104.5	143.0
TOTAL	104.5	147.4
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.0	0.1
12 MONTHS OR LONGER	2.4	2.1
TOTAL	2.4	$ 2.2
Number of debt securities, available-for-sale | security		3
Debt securities, available-for-sale, continuous unrealized loss position with credit loss	43.1	$ 98.4
Debt securities, available-for-sale, continuous unrealized loss position, accumulated loss, with credit loss	1.6	5.9
Net amount of recognized liabilities for repurchase agreements	0.2	1.2
Covered Bonds
FAIR VALUE
LESS THAN 12 MONTHS	0.0	0.0
12 MONTHS OR LONGER	126.7	213.2
TOTAL	126.7	213.2
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.0	0.0
12 MONTHS OR LONGER	3.1	9.7
TOTAL	3.1	9.7
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
FAIR VALUE
LESS THAN 12 MONTHS	645.3	0.0
12 MONTHS OR LONGER	917.6	2,477.0
TOTAL	1,562.9	2,477.0
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.7	0.0
12 MONTHS OR LONGER	44.9	105.8
TOTAL	45.6	$ 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	0.0	19.8
12 MONTHS OR LONGER	476.3	1,998.7
TOTAL	476.3	2,018.5
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.0	2.0
12 MONTHS OR LONGER	14.2	98.0
TOTAL	14.2	100.0
Commercial Mortgage-Backed
FAIR VALUE
LESS THAN 12 MONTHS	12.5	60.0
12 MONTHS OR LONGER	587.8	776.6
TOTAL	600.3	836.6
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.0	4.6
12 MONTHS OR LONGER	15.2	43.8
TOTAL	$ 15.2	$ 48.4