Loans Receivable - Overall Statistics for Loans Receivable Portfolio (Detail) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2015 USD ($) SecurityLoan	Dec. 31, 2014 USD ($) SecurityLoan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans | SecurityLoan	132	60
Principal Balance	$ 9,412,965	$ 4,462,897
Net book value	9,377,591	$ 4,428,500
Unfunded loan commitments	$ 738,300
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans | SecurityLoan	132	60
Principal Balance	$ 9,412,965	$ 4,462,897
Net book value	9,377,591	4,428,500
Unfunded loan commitments	$ 738,260	$ 513,229
Weighted-average maximum maturity (years)	3 years 3 months 18 days	3 years 10 months 24 days
Loans Receivable [Member] | LIBOR [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted-average cash coupon, rate	4.77%	4.36%
Weighted-average all-in yield, rate	5.09%	4.81%
Floating Rate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans | SecurityLoan	96
Principal Balance	$ 7,339,875
Net book value	7,303,478
Unfunded loan commitments	$ 733,540
Weighted-average cash coupon	L+4.18 %
Weighted-average all-in yield	L+4.56 %
Weighted-average maximum maturity (years)	3 years 4 months 24 days
Floating Rate [Member] | LIBOR [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted-average cash coupon, rate	4.18%
Weighted-average all-in yield, rate	4.55%
Fixed Rate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans | SecurityLoan	36
Principal Balance	$ 2,073,090
Net book value	2,074,113
Unfunded loan commitments	$ 4,720
Weighted-average maximum maturity (years)	2 years 10 months 24 days
Fixed Rate [Member] | LIBOR [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted-average cash coupon, rate	5.65%
Weighted-average all-in yield, rate	5.79%