Investment Securities Summary of Amortized Cost and Fair Values of Corporate Debt Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 2,493,408
Corporate debt securities	2,467,244
Corporate Debt Securities Issued by Financial Institutions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	84,899	$ 108,014
Corporate debt securities	85,004	105,407
Single-issuer Trust Preferred Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	43,795	43,746
Corporate debt securities	42,478	39,829
Subordinated Debt
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	29,069	46,231
Corporate debt securities	29,697	46,723
Senior Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	12,035	18,037
Corporate debt securities	12,407	18,433
Pooled Trust Preferred Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	0	0
Corporate debt securities	422	422
Other Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,988	4,002
Corporate debt securities	3,988	4,002
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	88,887	112,016
Corporate debt securities	$ 88,992	$ 109,409