Summary of Significant Accounting Policies - Short-term investments (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating cycle
Minimum liquidation period of assets in which significant working capital has been invested			1 year
Goodwill impairment	$ 2,716		$ 0	$ 2,716	$ 401
Income tax
Estimated net tax benefit of Tax Act			9,400
Comprehensive income
Other comprehensive income			$ 0	$ 0	$ 0
Cardinal Contractors
Operating cycle
Goodwill impairment		$ 400