Securities (Securities with Unrealized Losses) (Details) $ in Millions	Sep. 30, 2023 USD ($) Security	Dec. 31, 2022 USD ($) Security
FAIR VALUE
LESS THAN 12 MONTHS	$ 4,721.0	$ 15,665.4
12 MONTHS OR LONGER	17,339.1	5,769.4
TOTAL	22,060.1	21,434.8
UNREALIZED LOSSES
LESS THAN 12 MONTHS	32.4	727.7
12 MONTHS OR LONGER	873.8	321.5
TOTAL	906.2	1,049.2
U.S. Government
FAIR VALUE
LESS THAN 12 MONTHS	1,551.0	1,123.6
12 MONTHS OR LONGER	1,183.0	343.1
TOTAL	2,734.0	1,466.7
UNREALIZED LOSSES
LESS THAN 12 MONTHS	12.1	64.1
12 MONTHS OR LONGER	89.7	28.7
TOTAL	101.8	92.8
Obligations of States and Political Subdivisions
FAIR VALUE
LESS THAN 12 MONTHS	0.0	160.0
12 MONTHS OR LONGER	282.1	120.6
TOTAL	282.1	280.6
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.0	16.8
12 MONTHS OR LONGER	19.4	13.4
TOTAL	19.4	30.2
Government Sponsored Agency
FAIR VALUE
LESS THAN 12 MONTHS	2,855.0	7,631.4
12 MONTHS OR LONGER	7,865.9	2,737.7
TOTAL	10,720.9	10,369.1
UNREALIZED LOSSES
LESS THAN 12 MONTHS	18.1	262.1
12 MONTHS OR LONGER	320.0	89.5
TOTAL	338.1	351.6
Non-U.S. Government
FAIR VALUE
LESS THAN 12 MONTHS	0.0	235.4
12 MONTHS OR LONGER	246.9	124.6
TOTAL	246.9	360.0
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.0	17.3
12 MONTHS OR LONGER	24.1	10.3
TOTAL	24.1	27.6
Corporate Debt
FAIR VALUE
LESS THAN 12 MONTHS	12.4	427.3
12 MONTHS OR LONGER	229.9	130.3
TOTAL	242.3	557.6
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.3	14.6
12 MONTHS OR LONGER	9.9	2.8
TOTAL	$ 10.2	$ 17.4
Number of debt securities, available-for-sale | Security	3	3
Debt securities, available-for-sale, continuous unrealized loss position with credit loss	$ 81.5	$ 93.8
Debt securities, available-for-sale, continuous unrealized loss position, accumulated loss, with credit loss	6.9	9.5
Net amount of recognized liabilities for repurchase agreements	0.7	1.3
Covered Bonds
FAIR VALUE
LESS THAN 12 MONTHS	0.0	238.0
12 MONTHS OR LONGER	214.8	42.8
TOTAL	214.8	280.8
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.0	13.5
12 MONTHS OR LONGER	11.9	1.2
TOTAL	11.9	14.7
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
FAIR VALUE
LESS THAN 12 MONTHS	302.6	1,305.4
12 MONTHS OR LONGER	1,898.2	807.1
TOTAL	2,200.8	2,112.5
UNREALIZED LOSSES
LESS THAN 12 MONTHS	1.9	74.4
12 MONTHS OR LONGER	138.5	83.2
TOTAL	140.4	$ 157.6
Number of debt securities, available-for-sale | Security		1
Debt securities, available-for-sale, continuous unrealized loss position with credit loss		$ 68.3
Debt securities, available-for-sale, continuous unrealized loss position, accumulated loss, with credit loss		9.1
Other Asset-Backed
FAIR VALUE
LESS THAN 12 MONTHS	0.0	3,873.4
12 MONTHS OR LONGER	4,561.6	1,247.6
TOTAL	4,561.6	5,121.0
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.0	217.5
12 MONTHS OR LONGER	195.6	70.6
TOTAL	195.6	288.1
Commercial Mortgage-Backed
FAIR VALUE
LESS THAN 12 MONTHS	0.0	670.9
12 MONTHS OR LONGER	856.7	215.6
TOTAL	856.7	886.5
UNREALIZED LOSSES
LESS THAN 12 MONTHS	0.0	47.4
12 MONTHS OR LONGER	64.7	21.8
TOTAL	$ 64.7	$ 69.2