Securities - Remaining Contractual Maturity of Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Available-for-Sale, Amortized Cost
One year or less	$ 115,656
After one year to five years	157,219
After five years to ten years	792,796
After ten years	0
Securities that do not have a single contractual maturity date	2,159,614
Amortized Cost	3,225,285	$ 3,043,480
Available-for-Sale, Fair Value
One year or less	115,178
After one year to five years	156,570
After five years to ten years	789,520
After ten years	0
Securities that do not have a single contractual maturity date	2,151,640
Fair Value	3,212,908	3,096,408
Held-to-Maturity, Amortized Cost
One year or less	1,857
After one year to five years	4,495
After five years to ten years	1,953
After ten years	2,548
Securities that do not have a single contractual maturity date	0
Amortized Cost	10,853	$ 12,071
Held-to-Maturity, Fair Value
One year or less	1,790
After one year to five years	4,333
After five years to ten years	1,882
After ten years	2,456
Securities that do not have a single contractual maturity date	0
Fair Value	$ 10,461