Investment Securities Summary of Amortized Cost and Fair Values of Corporate Debt Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 2,664,163
Corporate debt securities	2,593,283
Corporate Debt Securities Issued by Financial Institutions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	88,455	$ 92,379
Corporate debt securities	87,914	93,335
Single-issuer Trust Preferred Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	21,779	31,335
Corporate debt securities	20,396	30,703
Subordinated Debt
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	54,649	49,013
Corporate debt securities	54,534	49,533
Senior Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	12,027	12,031
Corporate debt securities	12,109	12,392
Pooled Trust Preferred Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	0	0
Corporate debt securities	875	707
Other Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,956	3,974
Corporate debt securities	3,956	3,974
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	92,411	96,353
Corporate debt securities	$ 91,870	$ 97,309