Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Held-to-maturity, amortized cost, one year or less	$ 5,039
Held-to-maturity, amortized cost, after one through five years	17,346
Held-to-maturity, amortized cost, after five through ten years	5,561
Held-to-maturity, amortized cost, after ten years	2,742
Held-to-maturity, amortized cost, securities not due on a single maturity date	11,549
Amortized Cost	42,237	$ 289,194
Held-to-maturity, fair value, one year or less	5,057
Held-to-maturity, fair value, after one through five years	17,703
Held-to-maturity, fair value, after five through ten years	5,858
Held-to-maturity, fair value, after ten years	3,084
Held-to-maturity, fair value, securities not due on a single maturity date	11,600
Held-to-maturity, fair value, total	43,302	290,830
Available-for-sale, amortized cost, one year or less	7,599
Available-for-sale, amortized cost, after one through five years	70,442
Available-for-sale, amortized cost, after five through ten years	139,421
Available-for-sale, amortized cost, after ten years	632,504
Available-for-sale, amortized cost, securities not due on a single maturity date	1,332,130
Available-for-sale, amortized cost, other securities (no maturity)	145,010
Amortized Cost	2,327,106	2,187,623
Available-for-sale, fair value, one year or less	7,605
Available-for-sale, fair value, after one through five years	70,750
Available-for-sale, fair value, after five through ten years	142,101
Available-for-sale, fair value, after ten years	652,404
Available-for-sale, fair value, securities not due on a single maturity date	1,337,794
Available-for-sale, fair value, other securities (no maturity)	145,480
Available-for-sale, fair value, total	$ 2,356,134	$ 2,151,752