Loans Receivable - Principal Balance and Net Book Value of Loans Receivable Based on Internal Risk Ratings (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2014 SecurityLoan	Dec. 31, 2013 SecurityLoan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 3,906,226	$ 2,047,223
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans	55	31
Principal Balance	3,940,626	2,077,227
Net book value	3,906,226	2,047,223
Loans Receivable [Member] | Risk Rating 1-3 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	3,906,226	2,020,223
Loans Receivable [Member] | Risk Rating 6-8 [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	207,697	264,898
Net book value	204,463	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	207,697	227,350
Net book value	204,463	219,894
Subordinate Loans [Member] | Risk Rating 6-8 [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	53	26
Principal Balance	3,732,929	1,811,513
Net book value	3,701,763	1,800,329
Senior Loans [Member] | Risk Rating 1-3 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans	53	26
Principal Balance	3,732,929	1,811,513
Net book value	$ 3,701,763	$ 1,800,329