Loans Receivable, Net - Overall Statistics for Loans Receivable Portfolio (Detail) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($) SecurityLoan	Dec. 31, 2017 USD ($) SecurityLoan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	112	110
Net book value	$ 12,176,516	$ 10,056,732
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	112	110
Principal balance	$ 12,260,103	$ 10,108,226
Unfunded loan commitments	$ 3,046,350	$ 1,573,107
Weighted-average maximum maturity (years)	3 years 9 months 18 days	3 years 6 months
Loans Receivable [Member] | LIBOR [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted-average cash coupon, rate	5.61%	5.55%
Weighted-average all-in yield, rate	5.94%	5.95%
Net Book Value [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 12,176,516	$ 10,056,732