Investment Securities Summary of Amortized Cost and Fair Values of Corporate Debt Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Amortized Cost	$ 2,574,943
Estimated Fair Value	2,547,038
Corporate Debt Securities Issued by Financial Institutions [Member]
Amortized Cost	92,379	$ 108,014
Estimated Fair Value	93,335	105,407
Single-issuer trust preferred securities
Amortized Cost	31,335	43,746
Estimated Fair Value	30,703	39,829
Subordinated debt
Amortized Cost	49,013	46,231
Estimated Fair Value	49,533	46,723
Senior notes
Amortized Cost	12,031	18,037
Estimated Fair Value	12,392	18,433
Pooled trust preferred securities
Amortized Cost	0	0
Estimated Fair Value	707	422
Other corporate debt securities
Amortized Cost	3,974	4,002
Estimated Fair Value	3,974	4,002
Corporate debt securities
Amortized Cost	96,353	112,016
Estimated Fair Value	$ 97,309	$ 109,409