Securities (Remaining Maturity of Securities Available for Sale and Held to Maturity) (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Available for Sale - Cost
Due in One Year or Less	$ 9,976.3
Due After One Year Through Five Years	19,133.2
Due After Five Years Through Ten Years	9,932.4
Due After Ten Years	2,617.3
Total	41,659.2	$ 38,722.2
Available for Sale - Value
Due in One Year or Less	10,076.0
Due After One Year Through Five Years	19,537.8
Due After Five Years Through Ten Years	10,258.7
Due After Ten Years	2,678.5
Total	42,551.0	38,876.3
Held to Maturity - Amortized Cost
Due in One Year or Less	10,516.1
Due After One Year Through Five Years	5,813.7
Due After Five Years Through Ten Years	312.7
Due After Ten Years	111.5
Total	16,754.0	12,284.5
Held to Maturity - Fair Value
Due in One Year or Less	10,527.0
Due After One Year Through Five Years	5,859.4
Due After Five Years Through Ten Years	304.0
Due After Ten Years	56.1
Total	16,746.5	12,249.3
U.S. Government
Available for Sale - Cost
Due in One Year or Less	1,802.3
Due After One Year Through Five Years	1,834.8
Due After Five Years Through Ten Years	446.8
Due After Ten Years	0.0
Total	4,083.9	4,527.5
Available for Sale - Value
Due in One Year or Less	1,805.4
Due After One Year Through Five Years	1,876.1
Due After Five Years Through Ten Years	483.7
Due After Ten Years	0.0
Total	4,165.2	4,549.1
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
Total	99.0	138.8
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	138.8
Obligations of States and Political Subdivisions
Available for Sale - Cost
Due in One Year or Less	16.4
Due After One Year Through Five Years	210.9
Due After Five Years Through Ten Years	2,187.7
Due After Ten Years	50.2
Total	2,465.2	1,604.0
Available for Sale - Value
Due in One Year or Less	16.4
Due After One Year Through Five Years	222.8
Due After Five Years Through Ten Years	2,311.0
Due After Ten Years	50.3
Total	2,600.5	1,615.3
Held to Maturity - Amortized Cost
Due in One Year or Less	1.6
Due After One Year Through Five Years	1.0
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	2.6	10.1
Held to Maturity - Fair Value
Due in One Year or Less	1.6
Due After One Year Through Five Years	1.1
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	2.7	10.3
Government Sponsored Agency
Available for Sale - Cost
Due in One Year or Less	6,077.0
Due After One Year Through Five Years	9,520.9
Due After Five Years Through Ten Years	6,111.1
Due After Ten Years	2,453.6
Total	24,162.6	23,247.5
Available for Sale - Value
Due in One Year or Less	6,160.5
Due After One Year Through Five Years	9,669.4
Due After Five Years Through Ten Years	6,224.2
Due After Ten Years	2,514.7
Total	24,568.8	23,271.2
Held to Maturity - Amortized Cost
Due in One Year or Less	0.5
Due After One Year Through Five Years	1.5
Due After Five Years Through Ten Years	0.9
Due After Ten Years	0.5
Total	3.4	4.1
Held to Maturity - Fair Value
Due in One Year or Less	0.6
Due After One Year Through Five Years	1.6
Due After Five Years Through Ten Years	1.0
Due After Ten Years	0.5
Total	3.7	4.3
Non-U.S. Government
Available for Sale - Cost
Due in One Year or Less	737.9
Due After One Year Through Five Years	40.7
Due After Five Years Through Ten Years	158.2
Due After Ten Years	0.0
Total	936.8
Available for Sale - Value
Due in One Year or Less	738.2
Due After One Year Through Five Years	40.6
Due After Five Years Through Ten Years	158.8
Due After Ten Years	0.0
Total	937.6
Held to Maturity - Amortized Cost
Due in One Year or Less	7,291.5
Due After One Year Through Five Years	260.3
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	7,551.8	4,076.0
Held to Maturity - Fair Value
Due in One Year or Less	7,293.2
Due After One Year Through Five Years	266.5
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	7,559.7	4,078.8
Corporate Debt
Available for Sale - Cost
Due in One Year or Less	547.1
Due After One Year Through Five Years	2,001.6
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	2,548.7	2,378.9
Available for Sale - Value
Due in One Year or Less	552.9
Due After One Year Through Five Years	2,074.5
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	2,627.4	2,402.7
Held to Maturity - Amortized Cost
Due in One Year or Less	125.0
Due After One Year Through Five Years	405.3
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	530.3	405.1
Held to Maturity - Fair Value
Due in One Year or Less	125.2
Due After One Year Through Five Years	410.6
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	535.8	406.2
Covered Bonds
Available for Sale - Cost
Due in One Year or Less	24.4
Due After One Year Through Five Years	484.1
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	508.5	766.3
Available for Sale - Value
Due in One Year or Less	24.5
Due After One Year Through Five Years	492.3
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	516.8	769.9
Held to Maturity - Amortized Cost
Due in One Year or Less	1,228.1
Due After One Year Through Five Years	2,133.6
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	3,361.7	3,006.7
Held to Maturity - Fair Value
Due in One Year or Less	1,233.1
Due After One Year Through Five Years	2,153.0
Due After Five Years Through Ten Years	0.0
Due After Ten Years	0.0
Total	3,386.1	3,020.4
Certificates of Deposit
Held to Maturity - Amortized Cost
Due in One Year or Less	726.3
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	726.3	262.9
Held to Maturity - Fair Value
Due in One Year or Less	726.3
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	726.3	262.9
Sub-Sovereign, Supranational and Non-U.S. Agency Bonds
Available for Sale - Cost
Due in One Year or Less	334.9
Due After One Year Through Five Years	2,046.0
Due After Five Years Through Ten Years	105.5
Due After Ten Years	0.0
Total	2,486.4	2,091.3
Available for Sale - Value
Due in One Year or Less	335.1
Due After One Year Through Five Years	2,111.8
Due After Five Years Through Ten Years	105.9
Due After Ten Years	0.0
Total	2,552.8	2,127.6
Held to Maturity - Amortized Cost
Due in One Year or Less	832.7
Due After One Year Through Five Years	2,325.2
Due After Five Years Through Ten Years	232.6
Due After Ten Years	0.0
Total	3,390.5	3,285.4
Held to Maturity - Fair Value
Due in One Year or Less	837.1
Due After One Year Through Five Years	2,362.7
Due After Five Years Through Ten Years	233.2
Due After Ten Years	0.0
Total	3,433.0	3,305.0
Other Asset-Backed
Available for Sale - Cost
Due in One Year or Less	412.4
Due After One Year Through Five Years	2,658.0
Due After Five Years Through Ten Years	356.0
Due After Ten Years	95.9
Total	3,522.3	3,324.5
Available for Sale - Value
Due in One Year or Less	419.0
Due After One Year Through Five Years	2,690.0
Due After Five Years Through Ten Years	354.5
Due After Ten Years	95.9
Total	3,559.4	3,330.5
Held to Maturity - Amortized Cost
Due in One Year or Less	190.1
Due After One Year Through Five Years	435.4
Due After Five Years Through Ten Years	20.3
Due After Ten Years	0.0
Total	645.8	804.3
Held to Maturity - Fair Value
Due in One Year or Less	190.1
Due After One Year Through Five Years	435.8
Due After Five Years Through Ten Years	20.4
Due After Ten Years	0.0
Total	646.3	804.7
Commercial Mortgage-Backed
Available for Sale - Cost
Due in One Year or Less	16.6
Due After One Year Through Five Years	336.2
Due After Five Years Through Ten Years	567.1
Due After Ten Years	17.6
Total	937.5	769.9
Available for Sale - Value
Due in One Year or Less	16.7
Due After One Year Through Five Years	360.3
Due After Five Years Through Ten Years	620.6
Due After Ten Years	17.6
Total	1,015.2	797.7
Other
Available for Sale - Cost
Due in One Year or Less	7.3
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	7.3	9.0
Available for Sale - Value
Due in One Year or Less	7.3
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	7.3	9.0
Held to Maturity - Amortized Cost
Due in One Year or Less	21.3
Due After One Year Through Five Years	251.4
Due After Five Years Through Ten Years	58.9
Due After Ten Years	111.0
Total	442.6	291.1
Held to Maturity - Fair Value
Due in One Year or Less	20.8
Due After One Year Through Five Years	228.1
Due After Five Years Through Ten Years	49.4
Due After Ten Years	55.6
Total	$ 353.9	$ 217.9