Revenue from contracts with customers Contract balances (Details 2) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017
Revenue from Contract with Customer [Abstract]
Contract assets	$ 142,376	$ 142,376	$ 109,540
Change in contract assets		32,836
Contract liabilities - current	(98,470)	(98,470)	(84,123)
Change in contract liabilities - current		(14,347)
Contract liabilities - noncurrent	(399)	(399)	0
Change in contract liabilities - noncurrent		(399)
Net contract assets	43,507	43,507	$ 25,417
Change in net contract assets		18,090
Amounts included in contract liability at the beginning of the period	10,300	79,200
Amounts from performance obligations satisfied in prior periods	$ (8,700)	$ (3,700)