Securities - Amortized Cost and Estimated Fair Value of Investments in Debt Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Available for sale, Amortized Cost
Due in one year or less	$ 4,495
Due after one year through five years	26,705
Due after five years through ten years	175,953
Due after ten years	1,843,799
Total available-for-sale investment securities	2,050,952
MBS:	599,285
Amortized Cost	2,650,237	$ 2,437,513
Available for sale, Fair Value
Due in one year or less	4,532
Due after one year through five years	27,841
Due after five years through ten years	180,006
Due after ten years	1,918,344
Total available-for-sale investment securities	2,130,723
MBS:	622,381
Total	2,753,104	2,587,305
Held to maturity, Amortized Cost
Due in one year or less	120
Due after one year through five years	529
Due after five years through ten years	139
Due after ten years	0
Total held-to-maturity investment securities	788
Mortgage-backed securities	91,691
Amortized cost	92,479
Held to maturity, Fair Value
Due in one year or less	120
Due after one year through five years	531
Due after five years through ten years	140
Due after ten years	0
Total held-to-maturity investment securities	791
Mortgage-backed securities	97,244
Total	$ 98,035	$ 118,198