Notes Receivable - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Receivables [Abstract]
Notes receivable, past due	$ 0		$ 0
Notes receivable, unamortized discounts, balance	25,000,000		25,000,000		$ 25,000,000
Investment in impaired notes receivables	71,000,000		71,000,000		63,000,000
Notes receivable reserve representing an allowance for credit losses	55,000,000		55,000,000		50,000,000
Investment in impaired loans with no related allowance for credit losses	16,000,000		16,000,000		$ 13,000,000
Average investment in notes receivables	$ 69,000,000	$ 105,000,000	$ 67,000,000	$ 104,000,000