Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Held-to-maturity, amortized cost, one year or less	$ 50,800
Held-to-maturity, amortized cost, after one through five years	63,469
Held-to-maturity, amortized cost, after five through ten years	94,552
Held-to-maturity, amortized cost, after ten years	110,037
Held-to-maturity, amortized cost, securities not due on a single maturity date	14,645
Amortized Cost	333,503	$ 368,058
Held-to-maturity, fair value, one year or less	50,609
Held-to-maturity, fair value, after one through five years	63,414
Held-to-maturity, fair value, after five through ten years	94,503
Held-to-maturity, fair value, after ten years	112,289
Held-to-maturity, fair value, securities not due on a single maturity date	14,042
Held-to-maturity, fair value, total	334,857	373,298
Available-for-sale, amortized cost, one year or less	20
Available-for-sale, amortized cost, after one through five years	35,961
Available-for-sale, amortized cost, after five through ten years	22,549
Available-for-sale, amortized cost, after ten years	341,736
Available-for-sale, amortized cost, securities not due on a single maturity date	1,438,716
Available-for-sale, amortized cost, other securities (no maturity)	151,112
Amortized Cost	1,990,094	1,612,324
Available-for-sale, fair value, one year or less	20
Available-for-sale, fair value, after one through five years	35,125
Available-for-sale, fair value, after five through ten years	21,895
Available-for-sale, fair value, after ten years	334,161
Available-for-sale, fair value, securities not due on a single maturity date	1,395,231
Available-for-sale, fair value, other securities (no maturity)	152,212
Available-for-sale, fair value, total	$ 1,938,644	$ 1,589,517