Investment Securities Summary of Amortized Cost and Fair Values of Corporate Debt Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Amortized Cost	$ 2,479,382
Estimated Fair Value	2,463,259
Corporate Debt Securities Issued by Financial Institutions [Member]
Amortized Cost	96,301	$ 97,109
Estimated Fair Value	92,920	96,127
Single-issuer trust preferred securities
Amortized Cost	44,648	47,569
Estimated Fair Value	39,106	42,016
Subordinated debt
Amortized Cost	51,653	47,530
Estimated Fair Value	53,108	50,023
Pooled trust preferred securities
Amortized Cost	0	2,010
Estimated Fair Value	706	4,088
Other corporate debt securities
Amortized Cost	4,035	1,907
Estimated Fair Value	4,035	1,907
Corporate debt securities
Amortized Cost	100,336	99,016
Estimated Fair Value	$ 96,955	$ 98,034