Securities - Remaining Contractual Maturity of Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Available-for-Sale Securities, Amortized Cost Basis [Abstract]
One year or less	$ 87,379
After one year to five years	323,203
After five years to ten years	3,141
After ten years	0
Securities that do not have a single contractual maturity date	1,494,103
Amortized Cost	1,907,826	$ 1,957,584
Available-for-Sale Securities, Fair Value [Abstract]
One year or less	87,053
After one year to five years	322,000
After five years to ten years	3,130
After ten years	0
Securities that do not have a single contractual maturity date	1,472,026
Total	1,884,209	1,919,450
Held-to-Maturity Securities, Amortized Cost [Abstract]
One year or less	1,432
After one year to five years	5,831
After five years to ten years	2,243
After ten years	4,254
Securities that do not have a single contractual maturity date	0
Amortized Cost	13,760	22,291
Held-to-Maturity Securities, Fair Value [Abstract]
One year or less	1,250
After one year to five years	5,090
After five years to ten years	1,959
After ten years	3,714
Securities that do not have a single contractual maturity date	0
Total	$ 12,013	$ 18,212