Loans Receivable - Overall Statistics for Loans Receivable Portfolio (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014 SecurityLoan	Dec. 31, 2013 SecurityLoan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 4,428,500	$ 2,047,223
Unfunded loan commitments	513,200
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans	60	31
Principal balance	4,462,897	2,077,227
Net book value	4,428,500	2,047,223
Unfunded loan commitments	$ 513,229	$ 164,283
Weighted-average cash coupon	L+4.36 %	L+4.64 %
Weighted-average all-in yield	L+4.81 %	L+5.26 %
Weighted-average maximum maturity (years)	3 years 10 months 24 days	4 years 1 month 6 days