Revenue Recognition - Contract Balances with Contract Customers (details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Accounts receivable	$ 2,672	$ 15
Costs and estimated earnings in excess of billings on uncompleted contracts	0	680
Other current assets	1,221	1,015
Billings on uncompleted contracts in excess of related costs	446	5
Other current liabilities	10,063	10,142
Deferred revenue	5,000	0
Contracts with Customers
Accounts receivable	2,672	15
Costs and estimated earnings in excess of billings on uncompleted contracts	0	680
Other current assets	0	70
Billings on uncompleted contracts in excess of related costs	719	5
Other current liabilities	10,000	10,000
Deferred revenue	$ 5,000	$ 0