Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Held-to-maturity, amortized cost, one year or less	$ 4,205
Held-to-maturity, amortized cost, after one through five years	13,117
Held-to-maturity, amortized cost, after five through ten years	5,856
Held-to-maturity, amortized cost, after ten years	0
Held-to-maturity, amortized cost, securities not due on a single maturity date	24,297
Amortized Cost	47,475	$ 40,927
Held-to-maturity, fair value, one year or less	4,243
Held-to-maturity, fair value, after one through five years	13,698
Held-to-maturity, fair value, after five through ten years	6,126
Held-to-maturity, fair value, after ten years	0
Held-to-maturity, fair value, securities not due on a single maturity date	24,997
Held-to-maturity, fair value, total	49,064	41,855
Available-for-sale, amortized cost, one year or less	15,432
Available-for-sale, amortized cost, after one through five years	33,170
Available-for-sale, amortized cost, after five through ten years	192,972
Available-for-sale, amortized cost, after ten years	1,432,064
Available-for-sale, amortized cost, securities not due on a single maturity date	882,076
Available-for-sale, amortized cost, other securities (no maturity)	1,094
Amortized Cost	2,556,808	3,263,151
Available-for-sale, fair value, one year or less	15,538
Available-for-sale, fair value, after one through five years	33,657
Available-for-sale, fair value, after five through ten years	195,045
Available-for-sale, fair value, after ten years	1,458,119
Available-for-sale, fair value, securities not due on a single maturity date	903,687
Available-for-sale, fair value, other securities (no maturity)	1,242
Available-for-sale, fair value, total	$ 2,607,288	$ 3,288,343