Loans Receivable - Principal Balance and Net Book Value of Loans Receivable Based on Internal Risk Ratings (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
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
Loans Receivable [Member] | Risk Rating 1-3 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	4,428,500	2,020,223
Loans Receivable [Member] | Risk Rating 4-5 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value		27,000
Subordinate Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans	2	5
Principal balance	88,365	264,898
Net book value	87,914	246,894
Subordinate Loans [Member] | Risk Rating 1-3 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans	2	3
Principal balance	88,365	227,350
Net book value	87,914	219,894
Subordinate Loans [Member] | Risk Rating 4-5 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans		2
Principal balance		37,548
Net book value		27,000
Senior Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans	58	26
Principal balance	4,374,532	1,811,513
Net book value	4,340,586	1,800,329
Senior Loans [Member] | Risk Rating 1-3 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans	58	26
Principal balance	4,374,532	1,811,513
Net book value	$ 4,340,586	$ 1,800,329