Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Held-to-maturity, amortized cost, one year or less	$ 6,819
Held-to-maturity, amortized cost, after one through five years	7,680
Held-to-maturity, amortized cost, after five through ten years	9,734
Held-to-maturity, amortized cost, after ten years	847,602
Held-to-maturity, amortized cost, securities not due on a single maturity date	60,649
Amortized Cost	932,484	$ 335,946
Held-to-maturity, fair value, one year or less	6,884
Held-to-maturity, fair value, after one through five years	8,029
Held-to-maturity, fair value, after five through ten years	10,115
Held-to-maturity, fair value, after ten years	850,174
Held-to-maturity, fair value, securities not due on a single maturity date	60,394
Held-to-maturity, fair value, total	935,596	341,925
Available-for-sale, amortized cost, one year or less	11,777
Available-for-sale, amortized cost, after one through five years	51,854
Available-for-sale, amortized cost, after five through ten years	531,599
Available-for-sale, amortized cost, after ten years	1,949,938
Available-for-sale, amortized cost, securities not due on a single maturity date	3,996,823
Available-for-sale, amortized cost, other securities (no maturity)	1,031
Amortized Cost	6,543,022	3,397,043
Available-for-sale, fair value, one year or less	11,834
Available-for-sale, fair value, after one through five years	52,281
Available-for-sale, fair value, after five through ten years	537,748
Available-for-sale, fair value, after ten years	1,966,476
Available-for-sale, fair value, securities not due on a single maturity date	3,987,209
Available-for-sale, fair value, other securities (no maturity)	1,033
Available-for-sale, fair value, total	$ 6,556,581	$ 3,473,598