Loan Participations Sold, Net - Summary of Statistics for Loan Participations Sold (Parenthetical) (Detail) $ in Thousands, £ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 GBP (£)	Dec. 31, 2016 USD ($)
Participating Mortgage Loans [Line Items]
Guarantor obligations					£ 24.0	$ 29,600
Interest paid			$ 141,124	$ 123,564
Deferred financing costs	$ 17,462		17,462			15,272
Loan Participations Sold [Member]
Participating Mortgage Loans [Line Items]
Interest expense	4,000	$ 3,400	9,300	10,700
Interest paid	$ 2,600	$ 3,200	$ 7,700	$ 10,300
Deferred financing costs						$ 1,600