Securities (Securities with Unrealized Losses) (Details) $ in Millions	Jun. 30, 2023 USD ($) Security	Dec. 31, 2022 USD ($) Security
FAIR VALUE
LESS THAN 12 MONTHS	$ 3,979.9	$ 15,665.4
12 MONTHS OR LONGER	17,679.7	5,769.4
TOTAL	21,659.6	21,434.8
UNREALIZED LOSSES
LESS THAN 12 MONTHS	27.2	727.7
12 MONTHS OR LONGER	886.9	321.5
TOTAL	914.1	1,049.2
U.S. Government
FAIR VALUE
LESS THAN 12 MONTHS	865.4	1,123.6
12 MONTHS OR LONGER	1,182.8	343.1
TOTAL	2,048.2	1,466.7
UNREALIZED LOSSES
LESS THAN 12 MONTHS	10.4	64.1
12 MONTHS OR LONGER	88.6	28.7
TOTAL	99.0	92.8
Obligations of States and Political Subdivisions
FAIR VALUE
LESS THAN 12 MONTHS	0.0	160.0
12 MONTHS OR LONGER	289.2	120.6
TOTAL	289.2	280.6
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.0	16.8
12 MONTHS OR LONGER	21.0	13.4
TOTAL	21.0	30.2
Government Sponsored Agency
FAIR VALUE
LESS THAN 12 MONTHS	2,564.0	7,631.4
12 MONTHS OR LONGER	7,969.2	2,737.7
TOTAL	10,533.2	10,369.1
UNREALIZED LOSSES
LESS THAN 12 MONTHS	13.9	262.1
12 MONTHS OR LONGER	297.4	89.5
TOTAL	311.3	351.6
Non-U.S. Government
FAIR VALUE
LESS THAN 12 MONTHS	0.0	235.4
12 MONTHS OR LONGER	251.5	124.6
TOTAL	251.5	360.0
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.0	17.3
12 MONTHS OR LONGER	26.3	10.3
TOTAL	26.3	27.6
Corporate Debt
FAIR VALUE
LESS THAN 12 MONTHS	6.9	427.3
12 MONTHS OR LONGER	311.0	130.3
TOTAL	317.9	557.6
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.1	14.6
12 MONTHS OR LONGER	10.6	2.8
TOTAL	$ 10.7	$ 17.4
Number of debt securities, available-for-sale | Security	3	3
Debt securities, available-for-sale, continuous unrealized loss position with credit loss	$ 92.6	$ 93.8
Debt securities, available-for-sale, continuous unrealized loss position, accumulated loss, with credit loss	8.4	9.5
Net amount of recognized liabilities for repurchase agreements	1.3	1.3
Covered Bonds
FAIR VALUE
LESS THAN 12 MONTHS	0.0	238.0
12 MONTHS OR LONGER	256.3	42.8
TOTAL	256.3	280.8
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.0	13.5
12 MONTHS OR LONGER	12.8	1.2
TOTAL	12.8	14.7
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
FAIR VALUE
LESS THAN 12 MONTHS	542.4	1,305.4
12 MONTHS OR LONGER	1,798.7	807.1
TOTAL	2,341.1	2,112.5
UNREALIZED LOSSES
LESS THAN 12 MONTHS	2.8	74.4
12 MONTHS OR LONGER	139.8	83.2
TOTAL	$ 142.6	$ 157.6
Number of debt securities, available-for-sale | Security	1	1
Debt securities, available-for-sale, continuous unrealized loss position with credit loss	$ 68.6	$ 68.3
Debt securities, available-for-sale, continuous unrealized loss position, accumulated loss, with credit loss	8.8	9.1
Other Asset-Backed
FAIR VALUE
LESS THAN 12 MONTHS	0.0	3,873.4
12 MONTHS OR LONGER	4,748.4	1,247.6
TOTAL	4,748.4	5,121.0
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.0	217.5
12 MONTHS OR LONGER	226.2	70.6
TOTAL	226.2	288.1
Commercial Mortgage-Backed
FAIR VALUE
LESS THAN 12 MONTHS	1.2	670.9
12 MONTHS OR LONGER	872.6	215.6
TOTAL	873.8	886.5
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.0	47.4
12 MONTHS OR LONGER	64.2	21.8
TOTAL	$ 64.2	$ 69.2