Loans Receivable - Overall Statistics for Loans Receivable Portfolio (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2015 SecurityLoan	Dec. 31, 2014 SecurityLoan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans	60	60
Principal Balance	$ 4,977,468	$ 4,462,897
Net book value	4,943,383	4,428,500
Unfunded loan commitments	550,000
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans	60	60
Principal Balance	4,977,468	4,462,897
Net book value	4,943,383	4,428,500
Unfunded loan commitments	$ 550,012	$ 513,229
Weighted-average cash coupon	L+4.26 %	L+4.36 %
Weighted-average all-in yield	L+4.68 %	L+4.81 %
Weighted-average maximum maturity (years)	3 years 8 months 12 days	3 years 10 months 24 days