Securities Available for Sale and Held to Maturity - Contractual Maturities of Available for Sale and Held to Maturity Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Securities Available for Sale, Amortized Cost [Abstract]
Due in one year or less	$ 1,354
Due after one year through five years	16,721
Due after five years through ten years	16,191
Due after ten years	17,178
Total amortized cost, before mortgage-backed securities	51,444
Mortgage-backed securities	2,480,441
Securities Available for Sale, Amortized Cost	2,531,885	$ 1,959,773
Securities Available for Sale, Estimated Fair Value [Abstract]
Due in one year or less	1,376
Due after one year through five years	16,784
Due after five years through ten years	16,189
Due after ten years	17,506
Total fair value, before mortgage-backed securities	51,855
Mortgage-backed securities	2,496,884
Total	2,548,739	1,991,815
Securities Held to Maturity, Amortized Cost [Abstract]
Due in one year or less	7,660
Due after one year through five years	5,334
Due after five years through ten years	0
Due after ten years	0
Total amortized cost, before mortgage-backed securities	12,994
Mortgage-backed securities	420,018
Securities Held to Maturity, Amortized Cost	433,012	538,072
Securities Held to Maturity, Estimated Fair Value [Abstract]
Due in one year or less	7,687
Due after one year through five years	5,419
Due after five years through ten years	0
Due after ten years	0
Total fair value, before mortgage-backed securities	13,106
Mortgage-backed securities	439,182
Total	$ 452,288	$ 563,115