Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Held-to-maturity, amortized cost, one year or less	$ 50,503
Held-to-maturity, amortized cost, after one through five years	60,633
Held-to-maturity, amortized cost, after five through ten years	91,862
Held-to-maturity, amortized cost, after ten years	106,375
Held-to-maturity, amortized cost, securities not due on a single maturity date	13,933
Amortized Cost	323,306	$ 368,058
Held-to-maturity, fair value, one year or less	50,366
Held-to-maturity, fair value, after one through five years	60,313
Held-to-maturity, fair value, after five through ten years	91,364
Held-to-maturity, fair value, after ten years	107,560
Held-to-maturity, fair value, securities not due on a single maturity date	13,235
Held-to-maturity, fair value, total	322,838	373,298
Available-for-sale, amortized cost, one year or less	120
Available-for-sale, amortized cost, after one through five years	35,703
Available-for-sale, amortized cost, after five through ten years	22,485
Available-for-sale, amortized cost, after ten years	379,331
Available-for-sale, amortized cost, securities not due on a single maturity date	1,471,687
Available-for-sale, amortized cost, other securities (no maturity)	151,472
Amortized Cost	2,060,798	1,612,324
Available-for-sale, fair value, one year or less	119
Available-for-sale, fair value, after one through five years	34,714
Available-for-sale, fair value, after five through ten years	21,698
Available-for-sale, fair value, after ten years	367,466
Available-for-sale, fair value, securities not due on a single maturity date	1,419,626
Available-for-sale, fair value, other securities (no maturity)	154,191
Available-for-sale, fair value, total	$ 1,997,814	$ 1,589,517