Investment Securities (Asset backed securities) (Details)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	$ 2,404,500,000	$ 2,365,500,000	$ 2,306,800,000	$ 2,523,000,000	$ 2,465,900,000	$ 2,424,500,000
Level 2
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	1,841,300,000	$ 1,732,700,000	1,692,100,000	$ 1,761,600,000	$ 1,693,700,000	$ 1,753,100,000
Fixed rate CMBS
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	$ 85,000,000
Average basis points of subordination	20.00%
Floating rate CMBS
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	$ 41,100,000
GNMA
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	235,000,000		259,200,000
GNMA | Level 2
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	235,000,000		259,200,000
GNMA | Level 3
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	0		0
FNMA
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	1,700,000		1,900,000
FNMA | Level 2
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	1,700,000		1,900,000
FNMA | Level 3
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	0		0
FHLMC
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	0		0
FHLMC | Level 2
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	0		0
FHLMC | Level 3
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	0		0
Agency commercial mortgage-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	236,700,000		261,100,000
Agency commercial mortgage-backed securities | Level 2
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	236,700,000		261,100,000
Agency commercial mortgage-backed securities | Level 3
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	0		0
Residential
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	100,000,000		67,400,000
Residential | Level 2
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	100,000,000		67,400,000
Residential | Level 3
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	0		0
Commercial Mortgage Backed Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	126,100,000		96,700,000
Commercial Mortgage Backed Securities [Member] | Level 2
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	126,100,000		96,700,000
Commercial Mortgage Backed Securities [Member] | Level 3
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	0		0
Total Non-agency
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	226,100,000		164,100,000
Total Non-agency | Level 2
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	226,100,000		164,100,000
Total Non-agency | Level 3
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	0		0
Total mortgage-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	462,800,000		425,200,000
Total mortgage-backed securities | Level 2
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	462,800,000		425,200,000
Total mortgage-backed securities | Level 3
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	0		0
Credit card receivables
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	210,900,000		197,600,000
Credit card receivables | Level 2
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	210,900,000		197,600,000
Credit card receivables | Level 3
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	0		0
Vehicle receivables
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	191,100,000		145,800,000
Vehicle receivables | Level 2
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	191,100,000		145,800,000
Vehicle receivables | Level 3
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	0		0
Other
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	58,500,000		45,800,000
Other | Level 2
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	52,900,000		45,800,000
Other | Level 3
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	5,600,000		0
Total other asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	460,500,000		389,200,000
Total other asset-backed securities | Level 2
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	454,900,000		389,200,000
Total other asset-backed securities | Level 3
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	5,600,000		0
Total mortgage-backed and asset-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	923,300,000		814,400,000
Total mortgage-backed and asset-backed securities | Level 2
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	917,700,000		814,400,000
Total mortgage-backed and asset-backed securities | Level 3
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	5,600,000		$ 0
Non-Prime Residential Mortgage Backed Security
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	700,000
Non-Agency Residential Mortgage Backed Security [Domain]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	$ 38,800,000
Maximum | Commercial Mortgage Backed Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Percentage of underlying loans reported as non-performing	1.00%
Residential | Residential
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	$ 100,000,000
Residential, Subprime, Financing Receivable [Member] | Residential
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	$ 700,000
Residential, Subprime, Financing Receivable [Member] | Total mortgage-backed securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	1
Subordinate | Fixed rate CMBS
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	$ 46,100,000
Subordinate | Floating rate CMBS
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	41,100,000
Subordinate | Commercial Mortgage Backed Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	87,200,000
Subordinate | Residential | Residential
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	$ 0
Debt securities issued by corporations	0
Subordinate | Residential, Subprime, Financing Receivable [Member] | Residential
Schedule of Trading Securities and Other Trading Assets [Line Items]
Debt securities issued by corporations	$ 0