Contingencies and Commitments - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Rental expenses	$ 2,870	$ 2,546	$ 2,872
Remaining minimum amount committed	$ 106,381	$ 135,054