Loans Receivable - Principal Balance and Net Book Value of Loans Receivable Based on Internal Risk Ratings (Detail) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2015 USD ($) SecurityLoan	Dec. 31, 2014 USD ($) SecurityLoan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans | SecurityLoan	138	60
Principal Balance	$ 10,169,609	$ 4,462,897
Net book value	$ 10,131,323	$ 4,428,500
Risk Rating One [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans | SecurityLoan	9	5
Principal Balance	$ 970,624	$ 209,961
Net book value	$ 963,451	$ 209,112
Risk Rating Two [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans | SecurityLoan	87	44
Principal Balance	$ 6,626,108	$ 3,339,972
Net book value	$ 6,601,541	$ 3,313,906
Risk Rating 3 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of loans | SecurityLoan	42	11
Principal Balance	$ 2,572,877	$ 912,964
Net book value	$ 2,566,331	$ 905,482