Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Held-to-maturity, amortized cost, one year or less	$ 5,644
Held-to-maturity, amortized cost, after one through five years	7,676
Held-to-maturity, amortized cost, after five through ten years	27,094
Held-to-maturity, amortized cost, after ten years	1,419,732
Held-to-maturity, amortized cost, securities not due on a single maturity date	57,930
Amortized Cost	1,518,076	$ 335,946
Held-to-maturity, fair value, one year or less	5,678
Held-to-maturity, fair value, after one through five years	7,993
Held-to-maturity, fair value, after five through ten years	26,019
Held-to-maturity, fair value, after ten years	1,390,903
Held-to-maturity, fair value, securities not due on a single maturity date	57,323
Held-to-maturity, fair value, total	1,487,916	341,925
Available-for-sale, amortized cost, one year or less	8,385
Available-for-sale, amortized cost, after one through five years	53,515
Available-for-sale, amortized cost, after five through ten years	487,966
Available-for-sale, amortized cost, after ten years	1,853,165
Available-for-sale, amortized cost, securities not due on a single maturity date	4,434,981
Available-for-sale, amortized cost, other securities (no maturity)	731
Amortized Cost	6,838,743	3,397,043
Available-for-sale, fair value, one year or less	8,423
Available-for-sale, fair value, after one through five years	54,008
Available-for-sale, fair value, after five through ten years	492,654
Available-for-sale, fair value, after ten years	1,844,768
Available-for-sale, fair value, securities not due on a single maturity date	4,421,620
Available-for-sale, fair value, other securities (no maturity)	730
Available-for-sale, fair value, total	$ 6,822,203	$ 3,473,598