Offsetting of Assets and Liabilities (Offsetting of Derivative Liabilities and Securities Sold Under Agreements to Repurchase) (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Offsetting Liabilities [Line Items]
Gross recognized liabilities, total derivatives subject to a master netting arrangement	$ 1,652,200,000	$ 2,003,100,000
Gross amounts offset, total derivatives subject to a master netting arrangement	1,529,700,000	1,621,400,000
Net amounts presented, total derivatives subject to a master netting arrangement	122,500,000	381,700,000
Net amount, total derivatives subject to a master netting arrangement	122,500,000	381,700,000
Total Derivatives Not Subject to a Master Netting Arrangement	536,900,000	825,900,000
Gross recognized liabilities, total derivatives	2,189,100,000	2,829,000,000
Derivative Liabilities	659,400,000	1,207,600,000
Gross recognized liabilities, securities sold under agreements to repurchase	810,000,000	834,000,000
Net amounts presented, securities sold under agreements to repurchase	810,000,000	834,000,000
Gross amounts not offset, securities sold under agreements to repurchase	810,000,000	834,000,000
Other liabilities	2,200,000,000	2,400,000,000
Cash collateral possessed that was not offset	0	0
Foreign Exchange Contracts | Over the Counter Derivative
Offsetting Liabilities [Line Items]
Gross recognized liabilities, total derivatives subject to a master netting arrangement	1,522,200,000	1,889,200,000
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 arrangement	525,000,000	491,500,000
Net amount, total derivatives subject to a master netting arrangement	525,000,000	491,500,000
Interest Rate Swap Contracts | Over the Counter Derivative
Offsetting Liabilities [Line Items]
Gross recognized liabilities, total derivatives subject to a master netting arrangement	90,400,000	69,200,000
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 arrangement	70,600,000	55,000,000
Net amount, total derivatives subject to a master netting arrangement	70,600,000	55,000,000
Interest Rate Swap Contracts | Exchange Cleared
Offsetting Liabilities [Line Items]
Gross recognized liabilities, total derivatives subject to a master netting arrangement	8,300,000	14,300,000
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 arrangement	2,000,000	4,200,000
Net amount, total derivatives subject to a master netting arrangement	2,000,000	4,200,000
Other Financial Derivatives
Offsetting Liabilities [Line Items]
Gross recognized liabilities, total derivatives subject to a master netting arrangement	31,300,000	30,400,000
Net amounts presented, total derivatives subject to a master netting arrangement	31,300,000	30,400,000
Net amount, total derivatives subject to a master netting arrangement	31,300,000	30,400,000
Cross Product Netting Adjustment
Offsetting Liabilities [Line Items]
Gross amounts offset, total derivatives subject to a master netting arrangement	62,600,000	10,400,000
Cross Product Collateral Adjustment
Offsetting Liabilities [Line Items]
Gross amounts offset, total derivatives subject to a master netting arrangement	$ 443,800,000	$ 189,000,000