Securities - Remaining Contractual Maturity of Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Available-for-Sale, Amortized Cost
One year or less	$ 279,309
After one year to five years	206,291
After five years to ten years	561,776
After ten years	12,000
Securities that do not have a single contractual maturity date	2,157,241
Amortized Cost	3,216,617	$ 2,851,479
Available-for-Sale, Fair Value
One year or less	283,349
After one year to five years	209,275
After five years to ten years	569,902
After ten years	12,174
Securities that do not have a single contractual maturity date	2,205,184
Fair Value	3,279,884	$ 2,873,386
Held-to-Maturity, Amortized Cost
One year or less	1,094
After one year to five years	15,438
After five years to ten years	2,870
After ten years	2,791
Securities that do not have a single contractual maturity date	0
Amortized Cost	22,193
Held-to-Maturity, Fair Value
One year or less	1,101
After one year to five years	15,535
After five years to ten years	2,888
After ten years	2,808
Securities that do not have a single contractual maturity date	0
Fair Value	$ 22,332