Receivables - Receivables (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 3,039	$ 6,333
Allowance for bad debts	(26)	(26)
Receivables, net	3,013	6,307
Receivables and accrued interest
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	87	2,557
Notes receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total	$ 2,952	$ 3,776
Average interest rates	5.40%	5.40%