Securities Available for Sale and Held to Maturity - Contractual Maturities of Available for Sale and Held to Maturity Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Securities Available for Sale, Amortized Cost [Abstract]
Due in one year or less	$ 1,709
Due after one year through five years	189,184
Due after five years through ten years	94,838
Due after ten years	15,430
Total amortized cost, before mortgage-backed securities	301,161
Mortgage-backed securities	2,752,133
Securities Available for Sale, Amortized Cost	3,053,294	$ 1,959,773
Securities Available for Sale, Estimated Fair Value [Abstract]
Due in one year or less	1,729
Due after one year through five years	188,236
Due after five years through ten years	93,690
Due after ten years	15,673
Total fair value, before mortgage-backed securities	299,328
Mortgage-backed securities	2,758,277
Total	3,057,605	1,991,815
Securities Held to Maturity, Amortized Cost [Abstract]
Due in one year or less	6,161
Due after one year through five years	4,522
Due after five years through ten years	0
Due after ten years	0
Total amortized cost, before mortgage-backed securities	10,683
Mortgage-backed securities	384,222
Securities Held to Maturity, Amortized Cost	394,905	538,072
Securities Held to Maturity, Estimated Fair Value [Abstract]
Due in one year or less	6,186
Due after one year through five years	4,571
Due after five years through ten years	0
Due after ten years	0
Total fair value, before mortgage-backed securities	10,757
Mortgage-backed securities	400,375
Total	$ 411,132	$ 563,115