Securities - Remaining Contractual Maturity of Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Available-for-sale Securities, Amortized Cost [Abstract]
One year or less	$ 178,227
After one year to five years	175,643
After five years to ten years	277,094
After ten years	9
Securities that do not have a single contractual maturity date	2,236,051
Amortized Cost	2,867,024	$ 2,311,920
Fair Value
One year or less	180,121
After one year to five years	177,509
After five years to ten years	280,039
After ten years	9
Securities that do not have a single contractual maturity date	2,268,060
Fair Value	2,905,738	2,272,009
Held-to-maturity Securities, Amortized Cost [Abstract]
One year or less	9,002
After one year to five years	6,158
After five years to ten years	4,456
After ten years	2,950
Securities that do not have a single contractual maturity date	0
Amortized Cost	22,566	$ 10,176
Held-to-maturity Securities, Fair Value [Abstract]
One year or less	8,769
After one year to five years	5,999
After five years to ten years	4,341
After ten years	2,873
Securities that do not have a single contractual maturity date	0
Fair Value	$ 21,982