Securities (Remaining Maturity of Securities Available for Sale and Held to Maturity) (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Available for Sale - Amortized Cost
ONE YEAR OR LESS	$ 5,428.9
ONE TO FIVE YEARS	19,281.5
FIVE TO TEN YEARS	11,537.1
OVER TEN YEARS	1,726.2
AMORTIZED COST	37,973.7	$ 37,948.5
Available for Sale - Fair Value
ONE YEAR OR LESS	5,396.8
ONE TO FIVE YEARS	18,868.8
FIVE TO TEN YEARS	10,965.3
OVER TEN YEARS	1,630.0
FAIR VALUE	36,860.9	38,010.5
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	5,081.7
ONE TO FIVE YEARS	11,470.8
FIVE TO TEN YEARS	3,483.8
OVER TEN YEARS	1,187.2
Amortized Cost	21,223.5	23,564.5
Held to Maturity - Fair Value
ONE YEAR OR LESS	5,030.6
ONE TO FIVE YEARS	11,058.8
FIVE TO TEN YEARS	3,256.7
OVER TEN YEARS	1,053.0
Fair Value	20,399.1	23,315.4
U.S. Government
Available for Sale - Amortized Cost
ONE YEAR OR LESS	497.8
ONE TO FIVE YEARS	1,835.2
FIVE TO TEN YEARS	248.5
OVER TEN YEARS	0.0
AMORTIZED COST	2,581.5	2,406.2
Available for Sale - Fair Value
ONE YEAR OR LESS	499.3
ONE TO FIVE YEARS	1,822.5
FIVE TO TEN YEARS	225.2
OVER TEN YEARS	0.0
FAIR VALUE	2,547.0	2,426.1
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	49.0
ONE TO FIVE YEARS	0.0
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
Amortized Cost	49.0	47.0
Held to Maturity - Fair Value
ONE YEAR OR LESS	49.0
ONE TO FIVE YEARS	0.0
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
Fair Value	49.0	47.0
Obligations of States and Political Subdivisions
Available for Sale - Amortized Cost
ONE YEAR OR LESS	26.0
ONE TO FIVE YEARS	553.8
FIVE TO TEN YEARS	3,167.4
OVER TEN YEARS	152.5
AMORTIZED COST	3,899.7	3,841.0
Available for Sale - Fair Value
ONE YEAR OR LESS	26.1
ONE TO FIVE YEARS	542.0
FIVE TO TEN YEARS	2,942.7
OVER TEN YEARS	137.0
FAIR VALUE	3,647.8	3,876.1
Held to Maturity - Amortized Cost
Amortized Cost		0.8
Held to Maturity - Fair Value
Fair Value		0.8
Government Sponsored Agency
Available for Sale - Amortized Cost
ONE YEAR OR LESS	2,466.5
ONE TO FIVE YEARS	7,658.4
FIVE TO TEN YEARS	5,843.2
OVER TEN YEARS	1,415.8
AMORTIZED COST	17,383.9	18,092.1
Available for Sale - Fair Value
ONE YEAR OR LESS	2,436.8
ONE TO FIVE YEARS	7,540.7
FIVE TO TEN YEARS	5,613.0
OVER TEN YEARS	1,336.8
FAIR VALUE	16,927.3	18,075.6
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	885.0
ONE TO FIVE YEARS	2,077.2
FIVE TO TEN YEARS	2,048.6
OVER TEN YEARS	1,069.0
Amortized Cost	6,079.8	5,927.6
Held to Maturity - Fair Value
ONE YEAR OR LESS	831.8
ONE TO FIVE YEARS	1,940.8
FIVE TO TEN YEARS	1,906.7
OVER TEN YEARS	1,000.2
Fair Value	5,679.5	5,822.6
Non-U.S. Government
Available for Sale - Amortized Cost
ONE YEAR OR LESS	325.9
ONE TO FIVE YEARS	235.3
FIVE TO TEN YEARS	84.6
OVER TEN YEARS	0.0
AMORTIZED COST	645.8	383.4
Available for Sale - Fair Value
ONE YEAR OR LESS	325.8
ONE TO FIVE YEARS	220.1
FIVE TO TEN YEARS	75.5
OVER TEN YEARS	0.0
FAIR VALUE	621.4	374.0
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	1,515.3
ONE TO FIVE YEARS	1,227.3
FIVE TO TEN YEARS	208.3
OVER TEN YEARS	0.0
Amortized Cost	2,950.9	5,773.3
Held to Maturity - Fair Value
ONE YEAR OR LESS	1,515.5
ONE TO FIVE YEARS	1,186.4
FIVE TO TEN YEARS	199.2
OVER TEN YEARS	0.0
Fair Value	2,901.1	5,767.4
Corporate Debt
Available for Sale - Amortized Cost
ONE YEAR OR LESS	496.4
ONE TO FIVE YEARS	1,819.6
FIVE TO TEN YEARS	18.1
OVER TEN YEARS	3.8
AMORTIZED COST	2,337.9	2,319.8
Available for Sale - Fair Value
ONE YEAR OR LESS	496.4
ONE TO FIVE YEARS	1,775.5
FIVE TO TEN YEARS	16.2
OVER TEN YEARS	3.4
FAIR VALUE	2,291.5	2,341.7
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	229.8
ONE TO FIVE YEARS	686.7
FIVE TO TEN YEARS	15.7
OVER TEN YEARS	0.0
Amortized Cost	932.2	901.8
Held to Maturity - Fair Value
ONE YEAR OR LESS	230.0
ONE TO FIVE YEARS	669.3
FIVE TO TEN YEARS	14.2
OVER TEN YEARS	0.0
Fair Value	913.5	897.8
Covered Bonds
Available for Sale - Amortized Cost
ONE YEAR OR LESS	194.9
ONE TO FIVE YEARS	385.2
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
AMORTIZED COST	580.1	502.6
Available for Sale - Fair Value
ONE YEAR OR LESS	195.3
ONE TO FIVE YEARS	378.2
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
FAIR VALUE	573.5	505.6
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	564.7
ONE TO FIVE YEARS	1,832.7
FIVE TO TEN YEARS	544.5
OVER TEN YEARS	0.0
Amortized Cost	2,941.9	2,942.4
Held to Maturity - Fair Value
ONE YEAR OR LESS	565.8
ONE TO FIVE YEARS	1,803.2
FIVE TO TEN YEARS	511.1
OVER TEN YEARS	0.0
Fair Value	2,880.1	2,941.1
Certificates of Deposit
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	892.1
ONE TO FIVE YEARS	0.0
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
Amortized Cost	892.1	674.7
Held to Maturity - Fair Value
ONE YEAR OR LESS	892.1
ONE TO FIVE YEARS	0.0
FIVE TO TEN YEARS	0.0
OVER TEN YEARS	0.0
Fair Value	892.1	674.7
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Available for Sale - Amortized Cost
ONE YEAR OR LESS	522.1
ONE TO FIVE YEARS	2,012.0
FIVE TO TEN YEARS	412.2
OVER TEN YEARS	0.0
AMORTIZED COST	2,946.3	3,052.4
Available for Sale - Fair Value
ONE YEAR OR LESS	526.1
ONE TO FIVE YEARS	1,938.2
FIVE TO TEN YEARS	376.0
OVER TEN YEARS	0.0
FAIR VALUE	2,840.3	3,045.1
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	793.2
ONE TO FIVE YEARS	4,977.3
FIVE TO TEN YEARS	453.1
OVER TEN YEARS	0.0
Amortized Cost	6,223.6	6,098.0
Held to Maturity - Fair Value
ONE YEAR OR LESS	795.2
ONE TO FIVE YEARS	4,808.2
FIVE TO TEN YEARS	417.2
OVER TEN YEARS	0.0
Fair Value	6,020.6	6,032.3
Other Asset-Backed
Available for Sale - Amortized Cost
ONE YEAR OR LESS	891.7
ONE TO FIVE YEARS	3,657.9
FIVE TO TEN YEARS	1,395.5
OVER TEN YEARS	154.1
AMORTIZED COST	6,099.2	5,962.0
Available for Sale - Fair Value
ONE YEAR OR LESS	883.7
ONE TO FIVE YEARS	3,544.5
FIVE TO TEN YEARS	1,361.4
OVER TEN YEARS	152.8
FAIR VALUE	5,942.4	5,941.6
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	72.0
ONE TO FIVE YEARS	427.1
FIVE TO TEN YEARS	109.3
OVER TEN YEARS	0.0
Amortized Cost	608.4	682.6
Held to Maturity - Fair Value
ONE YEAR OR LESS	71.9
ONE TO FIVE YEARS	426.9
FIVE TO TEN YEARS	109.0
OVER TEN YEARS	0.0
Fair Value	607.8	683.6
Commercial Mortgage-Backed
Available for Sale - Amortized Cost
ONE YEAR OR LESS	7.6
ONE TO FIVE YEARS	1,124.1
FIVE TO TEN YEARS	367.6
OVER TEN YEARS	0.0
AMORTIZED COST	1,499.3	1,389.0
Available for Sale - Fair Value
ONE YEAR OR LESS	7.3
ONE TO FIVE YEARS	1,107.1
FIVE TO TEN YEARS	355.3
OVER TEN YEARS	0.0
FAIR VALUE	1,469.7	1,424.7
Other
Held to Maturity - Amortized Cost
ONE YEAR OR LESS	80.6
ONE TO FIVE YEARS	242.5
FIVE TO TEN YEARS	104.3
OVER TEN YEARS	118.2
Amortized Cost	545.6	516.3
Held to Maturity - Fair Value
ONE YEAR OR LESS	79.3
ONE TO FIVE YEARS	224.0
FIVE TO TEN YEARS	99.3
OVER TEN YEARS	52.8
Fair Value	$ 455.4	$ 448.1