Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Held-to-maturity, amortized cost, one year or less	$ 4,897
Held-to-maturity, amortized cost, after one through five years	15,972
Held-to-maturity, amortized cost, after five through ten years	7,387
Held-to-maturity, amortized cost, after ten years	1,875
Held-to-maturity, amortized cost, securities not due on a single maturity date	10,796
Amortized Cost	40,927	$ 289,194
Held-to-maturity, fair value, one year or less	4,919
Held-to-maturity, fair value, after one through five years	16,325
Held-to-maturity, fair value, after five through ten years	7,928
Held-to-maturity, fair value, after ten years	1,875
Held-to-maturity, fair value, securities not due on a single maturity date	10,808
Held-to-maturity, fair value, total	41,855	290,830
Available-for-sale, amortized cost, one year or less	494,432
Available-for-sale, amortized cost, after one through five years	72,949
Available-for-sale, amortized cost, after five through ten years	129,088
Available-for-sale, amortized cost, after ten years	825,155
Available-for-sale, amortized cost, securities not due on a single maturity date	1,738,584
Available-for-sale, amortized cost, other securities (no maturity)	167,938
Amortized Cost	3,428,146	2,187,623
Available-for-sale, fair value, one year or less	494,338
Available-for-sale, fair value, after one through five years	73,370
Available-for-sale, fair value, after five through ten years	131,267
Available-for-sale, fair value, after ten years	842,798
Available-for-sale, fair value, securities not due on a single maturity date	1,742,945
Available-for-sale, fair value, other securities (no maturity)	168,620
Available-for-sale, fair value, total	$ 3,453,338	$ 2,151,752