Securities (Remaining Maturity of Securities Available for Sale and Held to Maturity) (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Available for Sale - Amortized Cost
Due in One Year or Less	$ 5,299.6
Due After One Year Through Five Years	16,019.0
Due After Five Years Through Ten Years	13,524.2
Due After Ten Years	1,621.8
Total Fair Value	36,464.6	$ 41,155.7
Available for Sale - Fair Value
Due in One Year or Less	5,340.6
Due After One Year Through Five Years	16,243.2
Due After Five Years Through Ten Years	13,651.6
Due After Ten Years	1,628.6
Total	36,864.0	42,022.0
Held to Maturity - Amortized Cost
Due in One Year or Less	7,949.4
Due After One Year Through Five Years	8,910.1
Due After Five Years Through Ten Years	3,819.2
Due After Ten Years	976.6
Amortized Cost	21,655.3	17,791.1
Held to Maturity - Fair Value
Due in One Year or Less	7,916.7
Due After One Year Through Five Years	8,934.1
Due After Five Years Through Ten Years	3,801.5
Due After Ten Years	927.3
Total	21,579.6	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	1,872.2
Due After Five Years Through Ten Years	547.1
Due After Ten Years	0.0
Total Fair Value	2,519.3	2,728.8
Available for Sale - Fair Value
Due in One Year or Less	100.9
Due After One Year Through Five Years	1,904.7
Due After Five Years Through Ten Years	553.4
Due After Ten Years	0.0
Total	2,559.0	2,799.9
Held to Maturity - Amortized Cost
Due in One Year or Less	99.0
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	99.0	90.0
Held to Maturity - Fair Value
Due in One Year or Less	99.0
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	99.0	90.0
Obligations of States and Political Subdivisions
Available for Sale - Amortized Cost
Due in One Year or Less	23.7
Due After One Year Through Five Years	334.8
Due After Five Years Through Ten Years	2,858.1
Due After Ten Years	182.0
Total Fair Value	3,398.6	2,927.8
Available for Sale - Fair Value
Due in One Year or Less	24.0
Due After One Year Through Five Years	351.0
Due After Five Years Through Ten Years	2,933.4
Due After Ten Years	180.5
Total	3,488.9	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	2,601.7
Due After One Year Through Five Years	5,641.0
Due After Five Years Through Ten Years	8,047.7
Due After Ten Years	1,268.3
Total Fair Value	17,558.7	24,595.1
Available for Sale - Fair Value
Due in One Year or Less	2,621.0
Due After One Year Through Five Years	5,677.2
Due After Five Years Through Ten Years	8,071.1
Due After Ten Years	1,276.5
Total	17,645.8	24,956.7
Held to Maturity - Amortized Cost
Due in One Year or Less	1,432.3
Due After One Year Through Five Years	2,362.3
Due After Five Years Through Ten Years	1,845.3
Due After Ten Years	859.3
Amortized Cost	6,499.2	3.0
Held to Maturity - Fair Value
Due in One Year or Less	1,428.6
Due After One Year Through Five Years	2,355.3
Due After Five Years Through Ten Years	1,843.4
Due After Ten Years	855.6
Total	6,482.9	3.3
Non-U.S. Government
Available for Sale - Amortized Cost
Due in One Year or Less	340.7
Due After One Year Through Five Years	126.8
Due After Five Years Through Ten Years	161.0
Due After Ten Years	0.0
Total Fair Value	628.5
Available for Sale - Fair Value
Due in One Year or Less	340.8
Due After One Year Through Five Years	126.2
Due After Five Years Through Ten Years	156.8
Due After Ten Years	0.0
Total	623.8
Held to Maturity - Amortized Cost
Due in One Year or Less	3,015.7
Due After One Year Through Five Years	620.7
Due After Five Years Through Ten Years	165.8
Due After Ten Years	0.0
Amortized Cost	3,802.2	8,336.6
Held to Maturity - Fair Value
Due in One Year or Less	2,979.0
Due After One Year Through Five Years	625.3
Due After Five Years Through Ten Years	167.2
Due After Ten Years	0.0
Total	3,771.5	8,343.7
Corporate Debt
Available for Sale - Amortized Cost
Due in One Year or Less	359.6
Due After One Year Through Five Years	2,021.6
Due After Five Years Through Ten Years	17.9
Due After Ten Years	6.6
Total Fair Value	2,405.7	2,459.9
Available for Sale - Fair Value
Due in One Year or Less	363.6
Due After One Year Through Five Years	2,074.7
Due After Five Years Through Ten Years	18.0
Due After Ten Years	6.6
Total	2,462.9	2,539.6
Held to Maturity - Amortized Cost
Due in One Year or Less	4.1
Due After One Year Through Five Years	778.4
Due After Five Years Through Ten Years	98.4
Due After Ten Years	0.0
Amortized Cost	880.9	588.0
Held to Maturity - Fair Value
Due in One Year or Less	4.1
Due After One Year Through Five Years	782.4
Due After Five Years Through Ten Years	98.2
Due After Ten Years	0.0
Total	884.7	594.4
Covered Bonds
Available for Sale - Amortized Cost
Due in One Year or Less	105.2
Due After One Year Through Five Years	422.9
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total Fair Value	528.1	543.1
Available for Sale - Fair Value
Due in One Year or Less	105.3
Due After One Year Through Five Years	430.1
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	535.4	553.1
Held to Maturity - Amortized Cost
Due in One Year or Less	1,050.8
Due After One Year Through Five Years	1,465.2
Due After Five Years Through Ten Years	417.1
Due After Ten Years	0.0
Amortized Cost	2,933.1	3,184.6
Held to Maturity - Fair Value
Due in One Year or Less	1,055.3
Due After One Year Through Five Years	1,476.2
Due After Five Years Through Ten Years	416.0
Due After Ten Years	0.0
Total	2,947.5	3,208.9
Certificates of Deposit
Held to Maturity - Amortized Cost
Due in One Year or Less	906.2
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	906.2	807.2
Held to Maturity - Fair Value
Due in One Year or Less	906.2
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	906.2	807.2
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Available for Sale - Amortized Cost
Due in One Year or Less	520.6
Due After One Year Through Five Years	2,013.9
Due After Five Years Through Ten Years	489.0
Due After Ten Years	0.0
Total Fair Value	3,023.5	2,281.7
Available for Sale - Fair Value
Due in One Year or Less	522.8
Due After One Year Through Five Years	2,052.8
Due After Five Years Through Ten Years	482.9
Due After Ten Years	0.0
Total	3,058.5	2,345.8
Held to Maturity - Amortized Cost
Due in One Year or Less	1,051.6
Due After One Year Through Five Years	3,039.9
Due After Five Years Through Ten Years	1,241.6
Due After Ten Years	0.0
Amortized Cost	5,333.1	3,648.0
Held to Maturity - Fair Value
Due in One Year or Less	1,058.1
Due After One Year Through Five Years	3,058.0
Due After Five Years Through Ten Years	1,226.7
Due After Ten Years	0.0
Total	5,342.8	3,690.6
Other Asset-Backed
Available for Sale - Amortized Cost
Due in One Year or Less	1,234.1
Due After One Year Through Five Years	2,934.1
Due After Five Years Through Ten Years	915.5
Due After Ten Years	164.9
Total Fair Value	5,248.6	3,953.5
Available for Sale - Fair Value
Due in One Year or Less	1,248.2
Due After One Year Through Five Years	2,949.0
Due After Five Years Through Ten Years	915.9
Due After Ten Years	165.0
Total	5,278.1	3,997.5
Held to Maturity - Amortized Cost
Due in One Year or Less	266.8
Due After One Year Through Five Years	468.0
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Amortized Cost	734.8	677.0
Held to Maturity - Fair Value
Due in One Year or Less	267.3
Due After One Year Through Five Years	469.4
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	736.7	677.9
Other
Held to Maturity - Amortized Cost
Due in One Year or Less	121.9
Due After One Year Through Five Years	175.6
Due After Five Years Through Ten Years	51.0
Due After Ten Years	117.3
Amortized Cost	465.8	454.6
Held to Maturity - Fair Value
Due in One Year or Less	118.1
Due After One Year Through Five Years	167.5
Due After Five Years Through Ten Years	50.0
Due After Ten Years	71.7
Total	407.3	379.2
Commercial Mortgage-Backed
Available for Sale - Amortized Cost
Due in One Year or Less	14.0
Due After One Year Through Five Years	651.7
Due After Five Years Through Ten Years	487.9
Due After Ten Years	0.0
Total Fair Value	1,153.6	952.2
Available for Sale - Fair Value
Due in One Year or Less	14.0
Due After One Year Through Five Years	677.5
Due After Five Years Through Ten Years	520.1
Due After Ten Years	0.0
Total	$ 1,211.6	$ 1,031.8