Loans Receivable - Principal Balance and Net Book Value of Loans Receivable Based on Internal Risk Ratings (Detail) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2015 USD ($) SecurityLoan	Dec. 31, 2014 USD ($) SecurityLoan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans | SecurityLoan	132	60
Principal Balance	$ 9,412,965	$ 4,462,897
Net book value	$ 9,377,591	$ 4,428,500
Risk Rating One [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans | SecurityLoan	14	5
Principal Balance	$ 1,080,812	$ 209,961
Net book value	$ 1,074,105	$ 209,112
Risk Rating Two [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans | SecurityLoan	82	44
Principal Balance	$ 6,218,253	$ 3,339,972
Net book value	$ 6,196,357	$ 3,313,906
Risk Rating 3 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans | SecurityLoan	35	11
Principal Balance	$ 1,993,998	$ 912,964
Net book value	$ 1,987,472	$ 905,482
Risk Rating 4 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans | SecurityLoan	1
Principal Balance	$ 119,902
Net book value	$ 119,657