Offsetting of Assets and Liabilities (Offsetting of Derivative Assets and Securities Purchased Under Agreements to Resell) (Details) - USD ($)	Sep. 30, 2017	Dec. 31, 2016
Offsetting Assets [Line Items]
Gross recognized assets, total derivatives subject to a master netting arrangement	$ 2,576,500,000	$ 3,047,600,000
Gross amounts offset, total derivatives subject to a master netting arrangement	2,373,500,000	2,170,400,000
Net amounts presented, total derivatives subject to a master netting arrangements	203,000,000	877,200,000
Net amount, total derivatives not subject to a master netting arrangement	203,000,000	877,200,000
Total Derivatives Not Subject to a Master Netting Arrangement	(640,700,000)	(809,200,000)
Gross recognized assets, total derivatives	3,217,200,000	3,856,800,000
Net Amount	843,700,000	1,686,400,000
Gross recognized assets, securities purchased under agreement to resell	1,605,800,000	1,967,500,000
Net amounts presented, securities purchased under agreements to resell	1,605,800,000	1,967,500,000
Gross amounts not offset, securities purchased under agreements to resell	1,605,800,000	1,967,500,000
Other assets excluding derivatives and non financial instruments	3,800,000,000	3,300,000,000
Federal funds sold	65,000,000	6,800,000
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	2,459,400,000	2,800,400,000
Gross amounts offset, total derivatives subject to a master netting arrangement	1,702,000,000	1,651,900,000
Net amounts presented, total derivatives subject to a master netting arrangements	757,400,000	1,148,500,000
Net amount, total derivatives not subject to a master netting arrangement	757,400,000	1,148,500,000
Interest Rate Swap Contracts | Over the Counter Derivative
Offsetting Assets [Line Items]
Gross recognized assets, total derivatives subject to a master netting arrangement	3,000,000	129,800,000
Gross amounts offset, total derivatives subject to a master netting arrangement	12,400,000	18,200,000
Net amounts presented, total derivatives subject to a master netting arrangements	(9,400,000)	111,600,000
Net amount, total derivatives not subject to a master netting arrangement	(9,400,000)	111,600,000
Interest Rate Swap Contracts | Exchange Cleared
Offsetting Assets [Line Items]
Gross recognized assets, total derivatives subject to a master netting arrangement	114,100,000	117,400,000
Gross amounts offset, total derivatives subject to a master netting arrangement	22,100,000	21,800,000
Net amounts presented, total derivatives subject to a master netting arrangements	92,000,000	95,600,000
Net amount, total derivatives not subject to a master netting arrangement	92,000,000	95,600,000
Cross Product Netting Adjustment
Offsetting Assets [Line Items]
Gross amounts offset, total derivatives subject to a master netting arrangement	19,100,000	17,200,000
Cross Product Collateral Adjustment
Offsetting Assets [Line Items]
Gross amounts offset, total derivatives subject to a master netting arrangement	$ 617,900,000	$ 461,300,000