Investments in Securities - Unrealized Gain or Loss Position and Components of Carrying Value of Available for Sale Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Weighted average coupon rate	3.82%	3.83%
Principal Amount	$ 1,218,720	$ 1,248,910
Amortized Cost	1,248,920	1,280,872
Gross Unrealized Loss	(666)	(45)
Gross Unrealized Gain	29,101	107,018
Fair Value	1,277,355	1,387,845
Fannie Mae
Debt Securities, Available-for-sale [Line Items]
Principal Amount	1,050,660	1,063,403
Amortized Cost	1,074,388	1,088,209
Gross Unrealized Loss	(375)	(21)
Gross Unrealized Gain	28,317	99,138
Fair Value	1,102,330	1,187,326
Freddie Mac
Debt Securities, Available-for-sale [Line Items]
Principal Amount	155,761	172,550
Amortized Cost	161,932	179,385
Gross Unrealized Loss	(222)	(4)
Gross Unrealized Gain	775	7,797
Fair Value	162,485	187,178
Ginnie Mae
Debt Securities, Available-for-sale [Line Items]
Principal Amount	12,299	12,957
Amortized Cost	12,600	13,278
Gross Unrealized Loss	(69)	(20)
Gross Unrealized Gain	9	83
Fair Value	$ 12,540	$ 13,341