Investment Securities Summary of Amortized Cost and Fair Values of Corporate Debt Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 2,486,017
Corporate debt securities	2,497,138
Corporate Debt Securities Issued by Financial Institutions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	91,354	$ 96,301
Corporate debt securities	85,851	92,920
Single-issuer Trust Preferred Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	43,673	44,648
Corporate debt securities	35,851	39,106
Subordinated Debt
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	47,681	51,653
Corporate debt securities	49,294	53,108
Pooled Trust Preferred Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	0	0
Corporate debt securities	706	706
Other Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,035	4,035
Corporate debt securities	4,035	4,035
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	95,389	100,336
Corporate debt securities	$ 89,886	$ 96,955