Investment Securities - Contractual Maturities of Debt Securities Held to Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Amortized Cost
Due in one year	$ 28,174
Due after one year through five years	234,392
Due after five years through ten years	150,859
Due after ten years	209,280
Residential mortgage-backed securities	1,550,306
Amortized Cost	2,173,011	$ 2,336,095
Fair Value
Due in one year	28,290
Due after one year through five years	246,981
Due after five years through ten years	154,904
Due after ten years	207,782
Residential mortgage-backed securities	1,589,655
Total investment securities held to maturity	$ 2,227,612	$ 2,358,720