Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Held-to-maturity, amortized cost, one year or less	$ 6,118
Held-to-maturity, amortized cost, after one through five years	19,353
Held-to-maturity, amortized cost, after five through ten years	5,563
Held-to-maturity, amortized cost, after ten years	4,196
Held-to-maturity, amortized cost, securities not due on a single maturity date	12,225
Amortized Cost	47,455	$ 289,194
Held-to-maturity, fair value, one year or less	6,122
Held-to-maturity, fair value, after one through five years	19,716
Held-to-maturity, fair value, after five through ten years	5,785
Held-to-maturity, fair value, after ten years	4,748
Held-to-maturity, fair value, securities not due on a single maturity date	12,269
Held-to-maturity, fair value, total	48,640	290,830
Available-for-sale, amortized cost, one year or less	11,673
Available-for-sale, amortized cost, after one through five years	74,767
Available-for-sale, amortized cost, after five through ten years	151,019
Available-for-sale, amortized cost, after ten years	589,198
Available-for-sale, amortized cost, securities not due on a single maturity date	1,346,839
Available-for-sale, amortized cost, other securities (no maturity)	150,673
Amortized Cost	2,324,169	2,187,623
Available-for-sale, fair value, one year or less	11,681
Available-for-sale, fair value, after one through five years	75,019
Available-for-sale, fair value, after five through ten years	153,373
Available-for-sale, fair value, after ten years	605,410
Available-for-sale, fair value, securities not due on a single maturity date	1,345,760
Available-for-sale, fair value, other securities (no maturity)	151,144
Available-for-sale, fair value, total	$ 2,342,387	$ 2,151,752