Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Held-to-maturity, amortized cost, one year or less	$ 29,579
Held-to-maturity, amortized cost, after one through five years	60,564
Held-to-maturity, amortized cost, after five through ten years	88,770
Held-to-maturity, amortized cost, after ten years	96,935
Held-to-maturity, amortized cost, securities not due on a single maturity date	13,346
Amortized Cost	289,194	$ 368,058
Held-to-maturity, fair value, one year or less	29,515
Held-to-maturity, fair value, after one through five years	60,514
Held-to-maturity, fair value, after five through ten years	88,971
Held-to-maturity, fair value, after ten years	98,891
Held-to-maturity, fair value, securities not due on a single maturity date	12,939
Held-to-maturity, fair value, total	290,830	373,298
Available-for-sale, amortized cost, one year or less	120
Available-for-sale, amortized cost, after one through five years	35,518
Available-for-sale, amortized cost, after five through ten years	39,426
Available-for-sale, amortized cost, after ten years	407,815
Available-for-sale, amortized cost, securities not due on a single maturity date	1,552,487
Available-for-sale, amortized cost, other securities (no maturity)	152,257
Amortized Cost	2,187,623	1,612,324
Available-for-sale, fair value, one year or less	119
Available-for-sale, fair value, after one through five years	34,923
Available-for-sale, fair value, after five through ten years	39,360
Available-for-sale, fair value, after ten years	400,011
Available-for-sale, fair value, securities not due on a single maturity date	1,522,900
Available-for-sale, fair value, other securities (no maturity)	154,439
Available-for-sale, fair value, total	$ 2,151,752	$ 1,589,517