Investment Securities Summary of Amortized Cost and Fair Values of Corporate Debt Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Amortized Cost	$ 2,582,024
Estimated Fair Value	2,534,701
Corporate Debt Securities Issued by Financial Institutions [Member]
Amortized Cost	108,014	$ 96,301
Estimated Fair Value	105,407	92,920
Single-issuer trust preferred securities
Amortized Cost	43,746	44,648
Estimated Fair Value	39,829	39,106
Subordinated debt
Amortized Cost	46,231	39,610
Estimated Fair Value	46,723	40,779
Senior debt
Amortized Cost	18,037	12,043
Estimated Fair Value	18,433	12,329
Pooled trust preferred securities
Amortized Cost	0	0
Estimated Fair Value	422	706
Other corporate debt securities
Amortized Cost	4,002	4,035
Estimated Fair Value	4,002	4,035
Corporate debt securities
Amortized Cost	112,016	100,336
Estimated Fair Value	$ 109,409	$ 96,955