Loan Participations Sold, Net - Summary of Statistics for Loan Participations Sold (Parenthetical) (Detail) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Participating Mortgage Loans [Line Items]
Interest paid	$ 55,582,000	$ 39,287,000
Deferred financing costs	21,790,000		$ 16,681,000
Loan Participations Sold [Member]
Participating Mortgage Loans [Line Items]
Interest expense		1,500,000	83,400,000
Interest paid		$ 1,400,000	796,000,000
Deferred financing costs	$ 263,000,000		$ 291,000,000