Loans Receivable, Net - Principal Balance and Net Book Value of Loans Receivable Based on Internal Risk Ratings (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) SecurityLoan	Dec. 31, 2015 USD ($) SecurityLoan	Dec. 31, 2014 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	107	125
Net book value	$ 8,692,978	$ 9,077,007
Total Loan Exposure	$ 9,756,734	$ 10,148,126
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	107	125
Net Book Value [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 8,692,978	$ 9,077,007	$ 4,428,500
Risk Rating 1 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Exposure	$ 361,574	$ 925,443
Risk Rating 1 [Member] | Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	8	12
Risk Rating 1 [Member] | Net Book Value [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 361,100	$ 919,991
Risk Rating 2 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Exposure	$ 4,083,678	$ 6,316,890
Risk Rating 2 [Member] | Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	52	77
Risk Rating 2 [Member] | Net Book Value [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 4,011,992	$ 5,929,447
Risk Rating 3 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Exposure	$ 5,290,668	$ 2,792,510
Risk Rating 3 [Member] | Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	46	35
Risk Rating 3 [Member] | Net Book Value [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 4,299,026	$ 2,114,531
Risk Rating 4 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Loan Exposure	$ 20,814	$ 113,283
Risk Rating 4 [Member] | Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of Loans | SecurityLoan	1	1
Risk Rating 4 [Member] | Net Book Value [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value	$ 20,860	$ 113,038