Loans Receivable, Net - Overall Statistics for Loans Receivable Portfolio (Detail) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($) SecurityLoan	Dec. 31, 2016 USD ($) SecurityLoan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	111	107
Net book value	$ 9,637,152	$ 8,692,978
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	111	107
Principal balance	$ 9,681,055	$ 8,727,218
Unfunded loan commitments	$ 1,622,216	$ 882,472
Weighted-average maximum maturity (years)	3 years 4 months 24 days	3 years 2 months 12 days
Loans Receivable [Member] | LIBOR [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted-average cash coupon, rate	5.30%	5.01%
Weighted-average all-in yield, rate	5.68%	5.36%
Net Book Value [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 9,637,152	$ 8,692,978