Loans Receivable - Overall Statistics for Loans Receivable Portfolio (Detail) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2016 USD ($) SecurityLoan	Dec. 31, 2015 USD ($) SecurityLoan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 9,313,763	$ 9,077,007
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans | SecurityLoan	126	125
Principal Balance	$ 9,346,247	$ 9,108,361
Net book value	9,313,763	9,077,007
Unfunded loan commitments	$ 965,140	$ 700,658
Weighted-average maximum maturity (years)	3 years	3 years 1 month 6 days
Loans Receivable [Member] | LIBOR [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted-average cash coupon, rate	4.82%	4.84%
Weighted-average all-in yield, rate	5.18%	5.18%
Floating Rate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans | SecurityLoan	96	92
Principal Balance	$ 7,397,918	$ 7,098,180
Net book value	7,362,047	7,064,279
Unfunded loan commitments	$ 962,425	$ 696,276
Weighted-average maximum maturity (years)	3 years 2 months 12 days	3 years 3 months 18 days
Floating Rate [Member] | LIBOR [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted-average cash coupon, rate	4.08%	4.09%
Weighted-average all-in yield, rate	4.51%	4.49%
Fixed Rate [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans | SecurityLoan	30	33
Principal Balance	$ 1,948,329	$ 2,010,181
Net book value	1,951,716	2,012,728
Unfunded loan commitments	$ 2,715	$ 4,382
Weighted-average maximum maturity (years)	2 years 2 months 12 days	2 years 7 months 6 days
Fixed Rate [Member] | LIBOR [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted-average cash coupon, rate	5.64%	5.63%
Weighted-average all-in yield, rate	5.73%	5.78%