Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Investments, Debt and Equity Securities [Abstract]
Held-to-maturity, amortized cost, one year or less	$ 2,192
Held-to-maturity, amortized cost, after one through five years	9,261
Held-to-maturity, amortized cost, after five through ten years	384,518
Held-to-maturity, amortized cost, after ten years	2,171,211
Held-to-maturity, amortized cost, securities not due on a single maturity date	1,178,324
Amortized Cost	3,745,506	$ 3,761,094
Held-to-maturity, fair value, one year or less	2,128
Held-to-maturity, fair value, after one through five years	8,731
Held-to-maturity, fair value, after five through ten years	319,298
Held-to-maturity, fair value, after ten years	1,512,706
Held-to-maturity, fair value, securities not due on a single maturity date	1,005,348
Held-to-maturity, fair value, total	2,848,211	3,063,233
Available-for-sale, amortized cost, one year or less	69,877
Available-for-sale, amortized cost, after one through five years	159,285
Available-for-sale, amortized cost, after five through ten years	220,681
Available-for-sale, amortized cost, after ten years	996,002
Available-for-sale, amortized cost, securities not due on a single maturity date	2,444,803
Available-for-sale, amortized cost, other securities (no maturity)	262
Amortized Cost	3,890,910	4,331,413
Available-for-sale, fair value, one year or less	68,611
Available-for-sale, fair value, after one through five years	154,113
Available-for-sale, fair value, after five through ten years	189,896
Available-for-sale, fair value, after ten years	788,447
Available-for-sale, fair value, securities not due on a single maturity date	2,157,092
Available-for-sale, fair value, other securities (no maturity)	262
Available-for-sale, fair value, total	$ 3,358,421	$ 3,852,854