Subsequent Events - Additional Information (Details) kr / shares in Units, $ in Thousands, kr in Millions		12 Months Ended
	Feb. 13, 2018 NOK (kr) kr / shares shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) shares
Subsequent Event [Line Items]
Common stock, shares issued		28,330,460	27,150,674
Common stock, shares outstanding		28,330,460	27,150,674
Purchase price of acquisition, cash | $		$ 21,235	$ 2,306
UMS | Subsequent Event
Subsequent Event [Line Items]
Date of acquisition of business	Feb. 13, 2018
Offer price | kr / shares	kr 1.37
Common stock, shares issued	195,723,625
Common stock, shares outstanding	195,723,625
Purchase price of acquisition, cash | kr	kr 268.0
Minimum acceptance percentage to close the tender	90.00%
Threshold percentage to close the tender	50.00%
Tender offer threshold notice period to amend or withdraw recommendation	4 days
Break-fee payment by UMS upon withdrawing or amending recommendations or lapse of tender offer | kr	kr 12.5