Investment Securities - Contractual Maturities of Debt Securities Held to Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Amortized Cost
Due in one year	$ 113,188
Due after one year through five years	201,101
Due after five years through ten years	162,911
Due after ten years	201,468
Residential mortgage-backed securities	1,491,808
Amortized Cost	2,170,476	$ 2,336,095
Fair Value
Due in one year	113,628
Due after one year through five years	213,801
Due after five years through ten years	168,821
Due after ten years	200,018
Residential mortgage-backed securities	1,528,703
Total investment securities held to maturity	$ 2,224,971	$ 2,358,720