Subsequent Events - Additional Information (Detail) - USD ($) $ in Thousands			6 Months Ended	12 Months Ended
	Jul. 23, 2019	Jul. 22, 2019	Jun. 30, 2019	Dec. 31, 2018	Jul. 31, 2019
Subsequent Event [Line Items]
Maximum aggregate purchase price provided			$ 3,088,137	$ 3,108,661
Uncommitted amount of aggregate purchase price			$ 2,997,151	$ 2,325,246
Subsequent Event [Member] | Distressed mortgage loans [Member]
Subsequent Event [Line Items]
Fair value of performing and nonperforming loans agreed to sell					$ 71,000
JPMorgan Chase Bank, N.A. [Member] | Subsequent Event [Member] | Security Sold Under Agreements to Repurchase [Member]
Subsequent Event [Line Items]
Maximum aggregate purchase price provided	$ 1,000,000	$ 500,000
Uncommitted amount of aggregate purchase price	$ 950,000	$ 450,000