Loans Receivable - Principal Balance and Net Book Value of Loans Receivable Based on Internal Risk Ratings (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) SecurityLoan	Dec. 31, 2014 USD ($) SecurityLoan	Dec. 31, 2013 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 9,077,007	$ 4,428,500
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans | SecurityLoan	125	60
Principal Balance	$ 9,108,361	$ 4,462,897	$ 2,077,227
Net book value	$ 9,077,007	$ 4,428,500
Risk Rating 1 [Member] | Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans | SecurityLoan	12	5
Principal Balance	$ 925,443	$ 209,961
Net book value	$ 919,991	$ 209,112
Risk Rating 2 [Member] | Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans | SecurityLoan	77	44
Principal Balance	$ 5,948,922	$ 3,339,972
Net book value	$ 5,929,447	$ 3,313,906
Risk Rating 3 [Member] | Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans | SecurityLoan	35	11
Principal Balance	$ 2,120,713	$ 912,964
Net book value	$ 2,114,531	$ 905,482
Risk Rating 4 [Member] | Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans | SecurityLoan	1
Principal Balance	$ 113,283
Net book value	$ 113,038