Securities (Remaining Maturity of Securities Available for Sale and Held to Maturity) (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Available for Sale - Amortized Cost
Due in One Year or Less	$ 5,429.0
Due After One Year Through Five Years	19,368.0
Due After Five Years Through Ten Years	11,381.6
Due After Ten Years	1,562.9
Total Fair Value	37,741.5	$ 41,155.7
Available for Sale - Fair Value
Due in One Year or Less	5,463.6
Due After One Year Through Five Years	19,565.4
Due After Five Years Through Ten Years	11,442.4
Due After Ten Years	1,561.5
Total	38,032.9	42,022.0
Held to Maturity - Amortized Cost
Due in One Year or Less	5,808.9
Due After One Year Through Five Years	9,692.3
Due After Five Years Through Ten Years	4,243.0
Due After Ten Years	1,053.6
Amortized Cost	20,797.8	17,791.1
Held to Maturity - Fair Value
Due in One Year or Less	5,806.6
Due After One Year Through Five Years	9,683.2
Due After Five Years Through Ten Years	4,202.3
Due After Ten Years	1,000.7
Total	20,692.8	17,797.4
U.S. Government
Available for Sale - Amortized Cost
Due in One Year or Less	100.0
Due After One Year Through Five Years	2,168.7
Due After Five Years Through Ten Years	248.4
Due After Ten Years	0.0
Total Fair Value	2,517.1	2,728.8
Available for Sale - Fair Value
Due in One Year or Less	100.2
Due After One Year Through Five Years	2,209.2
Due After Five Years Through Ten Years	240.8
Due After Ten Years	0.0
Total	2,550.2	2,799.9
Held to Maturity - Amortized Cost
Due in One Year or Less	111.0
Due After One Year Through Five Years	0.0
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Amortized Cost	111.0	90.0
Held to Maturity - Fair Value
Due in One Year or Less	111.0
Due After One Year Through Five Years	0.0
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	111.0	90.0
Obligations of States and Political Subdivisions
Available for Sale - Amortized Cost
Due in One Year or Less	24.0
Due After One Year Through Five Years	399.4
Due After Five Years Through Ten Years	3,077.6
Due After Ten Years	181.1
Total Fair Value	3,682.1	2,927.8
Available for Sale - Fair Value
Due in One Year or Less	24.2
Due After One Year Through Five Years	417.5
Due After Five Years Through Ten Years	3,129.9
Due After Ten Years	178.5
Total	3,750.1	3,083.6
Held to Maturity - Amortized Cost
Due in One Year or Less	1.0
Due After One Year Through Five Years	0.0
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Amortized Cost	1.0	2.1
Held to Maturity - Fair Value
Due in One Year or Less	1.0
Due After One Year Through Five Years	0.0
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	1.0	2.2
Government Sponsored Agency
Available for Sale - Amortized Cost
Due in One Year or Less	2,878.7
Due After One Year Through Five Years	8,382.3
Due After Five Years Through Ten Years	6,022.9
Due After Ten Years	1,214.3
Total Fair Value	18,498.2	24,595.1
Available for Sale - Fair Value
Due in One Year or Less	2,892.9
Due After One Year Through Five Years	8,412.6
Due After Five Years Through Ten Years	6,020.7
Due After Ten Years	1,215.5
Total	18,541.7	24,956.7
Held to Maturity - Amortized Cost
Due in One Year or Less	1,266.9
Due After One Year Through Five Years	2,120.8
Due After Five Years Through Ten Years	1,883.8
Due After Ten Years	937.3
Amortized Cost	6,208.8	3.0
Held to Maturity - Fair Value
Due in One Year or Less	1,259.1
Due After One Year Through Five Years	2,106.2
Due After Five Years Through Ten Years	1,871.9
Due After Ten Years	929.2
Total	6,166.4	3.3
Non-U.S. Government
Available for Sale - Amortized Cost
Due in One Year or Less	0.0
Due After One Year Through Five Years	195.1
Due After Five Years Through Ten Years	82.2
Due After Ten Years	0.0
Total Fair Value	277.3	713.6
Available for Sale - Fair Value
Due in One Year or Less	0.0
Due After One Year Through Five Years	193.5
Due After Five Years Through Ten Years	79.9
Due After Ten Years	0.0
Total	273.4	714.0
Held to Maturity - Amortized Cost
Due in One Year or Less	1,543.0
Due After One Year Through Five Years	753.0
Due After Five Years Through Ten Years	496.8
Due After Ten Years	0.0
Amortized Cost	2,792.8	8,336.6
Held to Maturity - Fair Value
Due in One Year or Less	1,543.0
Due After One Year Through Five Years	757.0
Due After Five Years Through Ten Years	494.8
Due After Ten Years	0.0
Total	2,794.8	8,343.7
Corporate Debt
Available for Sale - Amortized Cost
Due in One Year or Less	382.9
Due After One Year Through Five Years	1,988.7
Due After Five Years Through Ten Years	18.0
Due After Ten Years	5.6
Total Fair Value	2,395.2	2,459.9
Available for Sale - Fair Value
Due in One Year or Less	387.6
Due After One Year Through Five Years	2,032.7
Due After Five Years Through Ten Years	17.9
Due After Ten Years	5.6
Total	2,443.8	2,539.6
Held to Maturity - Amortized Cost
Due in One Year or Less	25.1
Due After One Year Through Five Years	791.7
Due After Five Years Through Ten Years	129.0
Due After Ten Years	0.0
Amortized Cost	945.8	588.0
Held to Maturity - Fair Value
Due in One Year or Less	25.1
Due After One Year Through Five Years	794.3
Due After Five Years Through Ten Years	128.3
Due After Ten Years	0.0
Total	947.7	594.4
Covered Bonds
Available for Sale - Amortized Cost
Due in One Year or Less	174.6
Due After One Year Through Five Years	378.0
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total Fair Value	552.6	543.1
Available for Sale - Fair Value
Due in One Year or Less	175.4
Due After One Year Through Five Years	383.6
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	559.0	553.1
Held to Maturity - Amortized Cost
Due in One Year or Less	885.2
Due After One Year Through Five Years	1,572.2
Due After Five Years Through Ten Years	531.1
Due After Ten Years	0.0
Amortized Cost	2,988.5	3,184.6
Held to Maturity - Fair Value
Due in One Year or Less	887.7
Due After One Year Through Five Years	1,581.0
Due After Five Years Through Ten Years	528.6
Due After Ten Years	0.0
Total	2,997.3	3,208.9
Certificates of Deposit
Held to Maturity - Amortized Cost
Due in One Year or Less	777.8
Due After One Year Through Five Years	0.0
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Amortized Cost	777.8	807.2
Held to Maturity - Fair Value
Due in One Year or Less	777.8
Due After One Year Through Five Years	0.0
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	777.8	807.2
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Available for Sale - Amortized Cost
Due in One Year or Less	747.0
Due After One Year Through Five Years	1,885.6
Due After Five Years Through Ten Years	363.5
Due After Ten Years	0.0
Total Fair Value	2,996.1	2,281.7
Available for Sale - Fair Value
Due in One Year or Less	752.8
Due After One Year Through Five Years	1,910.6
Due After Five Years Through Ten Years	359.3
Due After Ten Years	0.0
Total	3,022.7	2,345.8
Held to Maturity - Amortized Cost
Due in One Year or Less	842.7
Due After One Year Through Five Years	3,850.4
Due After Five Years Through Ten Years	1,111.8
Due After Ten Years	0.0
Amortized Cost	5,804.9	3,648.0
Held to Maturity - Fair Value
Due in One Year or Less	847.7
Due After One Year Through Five Years	3,847.2
Due After Five Years Through Ten Years	1,089.5
Due After Ten Years	0.0
Total	5,784.4	3,690.6
Other Asset-Backed
Available for Sale - Amortized Cost
Due in One Year or Less	1,112.0
Due After One Year Through Five Years	3,218.7
Due After Five Years Through Ten Years	1,125.5
Due After Ten Years	161.9
Total Fair Value	5,618.1	3,953.5
Available for Sale - Fair Value
Due in One Year or Less	1,120.8
Due After One Year Through Five Years	3,227.5
Due After Five Years Through Ten Years	1,124.8
Due After Ten Years	161.9
Total	5,635.0	3,997.5
Held to Maturity - Amortized Cost
Due in One Year or Less	231.4
Due After One Year Through Five Years	424.5
Due After Five Years Through Ten Years	38.4
Due After Ten Years	0.0
Amortized Cost	694.3	677.0
Held to Maturity - Fair Value
Due in One Year or Less	231.8
Due After One Year Through Five Years	425.6
Due After Five Years Through Ten Years	38.5
Due After Ten Years	0.0
Total	695.9	677.9
Other
Held to Maturity - Amortized Cost
Due in One Year or Less	124.8
Due After One Year Through Five Years	179.7
Due After Five Years Through Ten Years	52.1
Due After Ten Years	116.3
Amortized Cost	472.9	454.6
Held to Maturity - Fair Value
Due in One Year or Less	122.4
Due After One Year Through Five Years	171.9
Due After Five Years Through Ten Years	50.7
Due After Ten Years	71.5
Total	416.5	379.2
Commercial Mortgage-Backed
Available for Sale - Amortized Cost
Due in One Year or Less	9.8
Due After One Year Through Five Years	751.5
Due After Five Years Through Ten Years	443.5
Due After Ten Years	0.0
Total Fair Value	1,204.8	952.2
Available for Sale - Fair Value
Due in One Year or Less	9.7
Due After One Year Through Five Years	778.2
Due After Five Years Through Ten Years	469.1
Due After Ten Years	0.0
Total	$ 1,257.0	$ 1,031.8