Loans Receivable - Overall Statistics for Loans Receivable Portfolio (Detail) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2016 USD ($) SecurityLoan	Dec. 31, 2015 USD ($) SecurityLoan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	106	125
Net book value	$ 8,347,712	$ 9,077,007
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	106	125
Principal balance	$ 8,381,679	$ 9,108,361
Unfunded loan commitments	$ 929,030	$ 700,658
Weighted-average maximum maturity (years)	3 years 2 months 12 days	3 years 1 month 6 days
Loans Receivable [Member] | LIBOR [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted-average cash coupon, rate	4.85%	4.84%
Weighted-average all-in yield, rate	5.22%	5.18%
Net Book Value [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 8,347,712	$ 9,077,007