Loan Participations Sold, Net - Summary of Statistics for Loan Participations Sold (Parenthetical) (Detail) £ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2017 GBP (£)
Participating Mortgage Loans [Line Items]
Guarantor obligations		$ 29,600,000		£ 24.0
Interest paid	$ 206,966,000	164,905,000	$ 127,429,000
Deferred financing costs	16,681,000	15,272,000
Loan Participations Sold [Member]
Participating Mortgage Loans [Line Items]
Interest expense	10,100,000	13,700,000
Interest paid	8,400,000	13,100,000
Deferred financing costs	$ 291,000	$ 1,600,000