Loans Receivable, Net - Overall Statistics for Loans Receivable Portfolio (Detail) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($) SecurityLoan	Dec. 31, 2017 USD ($) SecurityLoan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	106	110
Net book value	$ 11,081,716	$ 10,056,732
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	106	110
Principal balance	$ 11,140,975	$ 10,108,226
Unfunded loan commitments	$ 1,473,112	$ 1,573,107
Weighted-average maximum maturity (years)	3 years 8 months 12 days	3 years 6 months
Loans Receivable [Member] | LIBOR [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted-average cash coupon, rate	5.50%	5.55%
Weighted-average all-in yield, rate	5.88%	5.95%
Net Book Value [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 11,081,716	$ 10,056,732