Transactions with Related Parties - Summary of Management Fee Expense (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Related Party Transaction [Line Items]
Total management fee incurred during the period	$ 7,153	$ 7,175	$ 21,474	$ 21,510
PNMAC Capital Management LLC [Member]
Related Party Transaction [Line Items]
Total management fee incurred during the period	7,153	7,175	21,474	21,510
Average shareholders' equity amounts used to calculate base management fee expense	1,897,006	1,897,964	1,912,310	1,917,525
PNMAC Capital Management LLC [Member] | Base [Member]
Related Party Transaction [Line Items]
Total management fee incurred during the period	7,153	7,175	21,474	21,510
PNMAC Capital Management LLC [Member] | Performance incentive [Member]
Related Party Transaction [Line Items]
Total management fee incurred during the period	$ 0	$ 0	$ 0	$ 0