Receivables - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017
Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Due period of notes receivable as secured by deed of trust	3 years
Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Due period of notes receivable as secured by deed of trust	5 years