Offsetting of Assets and Liabilities (Offsetting of Derivative Assets and Securities Purchased Under Agreements to Resell) (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Offsetting Assets [Line Items]
Gross recognized assets, total derivatives subject to a master netting arrangement	$ 1,685,000,000	$ 2,203,300,000
Gross amounts offset, total derivatives subject to a master netting arrangement	1,160,800,000	1,860,000,000
Net amounts presented, total derivatives subject to a master netting arrangements	524,200,000	343,300,000
Net amount, total derivatives not subject to a master netting arrangement	524,200,000	343,300,000
Total Derivatives Not Subject to a Master Netting Arrangement	(457,500,000)	(450,800,000)
Gross recognized assets, total derivatives	2,142,500,000	2,654,100,000
Net Amount	981,700,000	794,100,000
Gross recognized assets, securities purchased under agreement to resell	1,504,000,000	1,303,300,000
Net amounts presented, securities purchased under agreements to resell	1,504,000,000	1,303,300,000
Gross amounts not offset, securities purchased under agreements to resell	1,504,000,000	1,303,300,000
Other assets excluding derivatives and non financial instruments	4,500,000,000	3,900,000,000
Federal funds sold	30,000,000	21,000,000
Cash collateral possessed that was not offset	0	0
Foreign Exchange Contracts | Over the Counter Derivative
Offsetting Assets [Line Items]
Gross recognized assets, total derivatives subject to a master netting arrangement	1,595,100,000	2,106,300,000
Gross amounts offset, total derivatives subject to a master netting arrangement	997,200,000	1,397,700,000
Net amounts presented, total derivatives subject to a master netting arrangements	597,900,000	708,600,000
Net amount, total derivatives not subject to a master netting arrangement	597,900,000	708,600,000
Interest Rate Swap Contracts | Over the Counter Derivative
Offsetting Assets [Line Items]
Gross recognized assets, total derivatives subject to a master netting arrangement	83,400,000	86,900,000
Gross amounts offset, total derivatives subject to a master netting arrangement	19,800,000	14,200,000
Net amounts presented, total derivatives subject to a master netting arrangements	63,600,000	72,700,000
Net amount, total derivatives not subject to a master netting arrangement	63,600,000	72,700,000
Interest Rate Swap Contracts | Exchange Cleared
Offsetting Assets [Line Items]
Gross recognized assets, total derivatives subject to a master netting arrangement	6,300,000	10,100,000
Gross amounts offset, total derivatives subject to a master netting arrangement	6,300,000	10,100,000
Net amounts presented, total derivatives subject to a master netting arrangements	0	0
Net amount, total derivatives not subject to a master netting arrangement	0	0
Other Financial Derivatives
Offsetting Assets [Line Items]
Gross recognized assets, total derivatives subject to a master netting arrangement	200,000
Gross amounts offset, total derivatives subject to a master netting arrangement	0
Net amounts presented, total derivatives subject to a master netting arrangements	200,000
Net amount, total derivatives not subject to a master netting arrangement	200,000
Cross Product Netting Adjustment
Offsetting Assets [Line Items]
Gross amounts offset, total derivatives subject to a master netting arrangement	62,600,000	10,400,000
Cross Product Collateral Adjustment
Offsetting Assets [Line Items]
Gross amounts offset, total derivatives subject to a master netting arrangement	$ 74,900,000	$ 427,600,000