Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Held-to-maturity, amortized cost, one year or less	$ 5,821
Held-to-maturity, amortized cost, after one through five years	10,708
Held-to-maturity, amortized cost, after five through ten years	5,227
Held-to-maturity, amortized cost, after ten years	291,836
Held-to-maturity, amortized cost, securities not due on a single maturity date	22,354
Amortized Cost	335,946	$ 40,927
Held-to-maturity, fair value, one year or less	5,895
Held-to-maturity, fair value, after one through five years	11,199
Held-to-maturity, fair value, after five through ten years	2,937
Held-to-maturity, fair value, after ten years	298,857
Held-to-maturity, fair value, securities not due on a single maturity date	23,037
Held-to-maturity, fair value, total	341,925	41,855
Available-for-sale, amortized cost, one year or less	14,017
Available-for-sale, amortized cost, after one through five years	30,634
Available-for-sale, amortized cost, after five through ten years	229,042
Available-for-sale, amortized cost, after ten years	1,747,650
Available-for-sale, amortized cost, securities not due on a single maturity date	1,374,769
Available-for-sale, amortized cost, other securities (no maturity)	931
Amortized Cost	3,397,043	3,263,151
Available-for-sale, fair value, one year or less	14,089
Available-for-sale, fair value, after one through five years	31,071
Available-for-sale, fair value, after five through ten years	232,003
Available-for-sale, fair value, after ten years	1,800,568
Available-for-sale, fair value, securities not due on a single maturity date	1,394,936
Available-for-sale, fair value, other securities (no maturity)	931
Available-for-sale, fair value, total	$ 3,473,598	$ 3,288,343