Loans Receivable - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted-average risk rating on loan exposure	2.5	2.2
Impaired loans	$ 0	$ 0
Maturity date	Feb. 28, 2017
Partial repayment of loan on sale of collateralized assets	$ 102,600,000
Net book value of loan after partial repayment	8,347,712,000	9,077,007,000
Net Book Value [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value of loan after partial repayment	8,347,712,000	9,077,007,000
Risk Rating 2 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Principal Balance of collateralized loan	113,300,000
Risk Rating 2 [Member] | Net Book Value [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value of loan after partial repayment	3,882,247,000	$ 5,929,447,000
Risk Rating 2 [Member] | Net Book Value [Member] | Extended Maturity [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Net book value of loan after partial repayment	$ 10,200,000