Securities - Remaining Contractual Maturity of Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Available-for-sale Securities, Amortized Cost [Abstract]
One year or less	$ 113,814
After one year to five years	171,155
After five years to ten years	199,166
After ten years	0
Securities that do not have a single contractual maturity date	1,766,657
Amortized Cost	2,250,792
Available-for-sale Securities, Fair Value [Abstract]
One year or less	111,859
After one year to five years	168,215
After five years to ten years	195,744
After ten years	0
Securities that do not have a single contractual maturity date	1,703,592
Fair Value	2,179,410
Held-to-maturity Securities, Amortized Cost [Abstract]
One year or less	1,631
After one year to five years	4,915
After five years to ten years	2,176
After ten years	3,951
Securities that do not have a single contractual maturity date	0
Amortized Cost	12,673	$ 13,760
Held-to-maturity Securities, Fair Value [Abstract]
One year or less	1,353
After one year to five years	4,078
After five years to ten years	1,806
After ten years	3,278
Securities that do not have a single contractual maturity date	0
Fair Value	$ 10,515