Fair Value of Financial Instruments - Fair Values Measured on Recurring Basis (Detail) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage-backed securities	$ 7,804,259	$ 8,172,182
Derivative assets	270	10,004
Derivative liabilities	2,951	807
Commercial loan, held-for-investment	23,515	23,098
Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage-backed securities	7,804,259	8,172,182
Derivative assets	270	10,004
Other assets	35,991	39,506
Total assets	7,840,520	8,221,692
Derivative liabilities	14,356	6,344
Total liabilities	14,356	6,344
Recurring | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage-backed securities	0	0
Derivative assets	0	0
Other assets	0	0
Total assets	0	0
Derivative liabilities	0	0
Total liabilities	0	0
Recurring | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage-backed securities	7,804,259	8,172,182
Derivative assets	270	10,004
Other assets	0	0
Total assets	7,804,529	8,182,186
Derivative liabilities	14,356	6,344
Total liabilities	14,356	6,344
Recurring | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Mortgage-backed securities	0	0
Derivative assets	0	0
Other assets	23,515	23,098
Total assets	23,515	23,098
Derivative liabilities	0	0
Total liabilities	0	0
Recurring | NAV as a practical expedient
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
NAV as a practical expedient	$ 12,476	$ 16,408