Investment Securities - Securities Available for Sale Classified by Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Amortized Cost
Due in one year or less, amortized cost	$ 502,465	$ 504,851
Due after one year through five years, amortized cost	18,679	38,710
Due after five years through ten years, amortized cost	77,433	53,598
Due after ten years, amortized cost	102,266	11,102
Subtotal, amortized cost	700,843	608,261
Amortized Cost	1,161,098	1,001,034
Fair Value
Due in one year or less, fair value	502,489	504,890
Due after one year through five years, fair value	19,167	38,623
Due after five years through ten years, fair value	77,681	53,457
Due after ten years, fair value	102,507	11,180
Subtotal, fair value	701,844	608,150
Fair Value	1,174,964	1,005,984
Mortgage-backed securities
Amortized Cost
Securities with no stated maturity, amortized cost	460,255	392,773
Amortized Cost	441,575	360,977
Fair Value
Securities with no stated maturity, fair value	473,120	397,834
Fair Value	$ 453,857	$ 366,002