Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Held-to-maturity, amortized cost, one year or less	$ 18,407
Held-to-maturity, amortized cost, after one through five years	20,419
Held-to-maturity, amortized cost, after five through ten years	5,564
Held-to-maturity, amortized cost, after ten years	4,198
Held-to-maturity, amortized cost, securities not due on a single maturity date	12,847
Amortized Cost	61,435	$ 289,194
Held-to-maturity, fair value, one year or less	18,397
Held-to-maturity, fair value, after one through five years	20,624
Held-to-maturity, fair value, after five through ten years	5,691
Held-to-maturity, fair value, after ten years	4,627
Held-to-maturity, fair value, securities not due on a single maturity date	12,617
Held-to-maturity, fair value, total	61,956	290,830
Available-for-sale, amortized cost, one year or less	7,375
Available-for-sale, amortized cost, after one through five years	74,428
Available-for-sale, amortized cost, after five through ten years	123,088
Available-for-sale, amortized cost, after ten years	535,761
Available-for-sale, amortized cost, securities not due on a single maturity date	1,362,950
Available-for-sale, amortized cost, other securities (no maturity)	146,501
Amortized Cost	2,250,103	2,187,623
Available-for-sale, fair value, one year or less	7,372
Available-for-sale, fair value, after one through five years	74,208
Available-for-sale, fair value, after five through ten years	124,101
Available-for-sale, fair value, after ten years	541,815
Available-for-sale, fair value, securities not due on a single maturity date	1,345,677
Available-for-sale, fair value, other securities (no maturity)	146,938
Available-for-sale, fair value, total	$ 2,240,111	$ 2,151,752