Securities - Remaining Contractual Maturity of Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Available-for-sale Securities, Amortized Cost [Abstract]
One year or less	$ 133,657
After one year to five years	167,113
After five years to ten years	267,707
After ten years	9
Securities that do not have a single contractual maturity date	2,199,752
Amortized Cost	2,768,238	$ 2,311,920
Fair Value
One year or less	134,761
After one year to five years	168,493
After five years to ten years	269,918
After ten years	9
Securities that do not have a single contractual maturity date	2,220,135
Fair Value	2,793,316	2,272,009
Held-to-maturity Securities, Amortized Cost [Abstract]
One year or less	8,736
After one year to five years	5,982
After five years to ten years	5,531
After ten years	3,028
Securities that do not have a single contractual maturity date	0
Amortized Cost	23,277	$ 10,176
Held-to-maturity Securities, Fair Value [Abstract]
One year or less	8,409
After one year to five years	5,758
After five years to ten years	5,324
After ten years	2,914
Securities that do not have a single contractual maturity date	0
Fair Value	$ 22,405