Investment Securities - Contractual Maturities of Debt Securities Held to Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Amortized Cost
Due in one year	$ 22,683
Due after one year through five years	231,112
Due after five years through ten years	66,064
Due after ten years	191,844
Residential mortgage-backed securities	2,072,700
Amortized Cost	2,584,403	$ 2,173,011
Fair Value
Due in one year	22,845
Due after one year through five years	239,893
Due after five years through ten years	67,333
Due after ten years	189,643
Residential mortgage-backed securities	2,076,597
Total	$ 2,596,311	$ 2,227,612