Securities Available for Sale and Held to Maturity - Contractual Maturities of Available for Sale and Held to Maturity Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Securities Available for Sale, Amortized Cost [Abstract]
Due in one year or less	$ 1,479
Due after one year through five years	1,510
Due after five years through ten years	1,967
Due after ten years	18,004
Total amortized cost, before mortgage-backed securities	22,960
Mortgage-backed securities	2,304,474
Securities Available for Sale, Amortized Cost	2,327,434	$ 1,959,773
Securities Available for Sale, Estimated Fair Value [Abstract]
Due in one year or less	1,507
Due after one year through five years	1,554
Due after five years through ten years	1,928
Due after ten years	18,158
Total fair value, before mortgage-backed securities	23,147
Mortgage-backed securities	2,314,529
Total	2,337,676	1,991,815
Securities Held to Maturity, Amortized Cost [Abstract]
Due in one year or less	19,212
Due after one year through five years	7,342
Due after five years through ten years	0
Due after ten years	0
Total amortized cost, before mortgage-backed securities	26,554
Mortgage-backed securities	467,184
Securities Held to Maturity, Amortized Cost	493,738	538,072
Securities Held to Maturity, Estimated Fair Value [Abstract]
Due in one year or less	19,275
Due after one year through five years	7,428
Due after five years through ten years	0
Due after ten years	0
Total fair value, before mortgage-backed securities	26,703
Mortgage-backed securities	488,593
Total	$ 515,296	$ 563,115