Investment Securities Summary of Amortized Cost and Fair Values of Corporate Debt Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 2,569,641
Corporate debt securities	2,548,457
Corporate Debt Securities Issued by Financial Institutions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	88,366	$ 108,014
Corporate debt securities	88,988	105,407
Single-issuer Trust Preferred Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	39,186	43,746
Corporate debt securities	38,251	39,829
Subordinated Debt
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	37,147	46,231
Corporate debt securities	37,859	46,723
Senior Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	12,033	18,037
Corporate debt securities	12,456	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	3,989	4,002
Corporate debt securities	3,989	4,002
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	92,355	112,016
Corporate debt securities	$ 92,977	$ 109,409