Revenue from contracts with customers Contract balances (Details 2) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Revenue from Contract with Customer [Abstract]
Contract assets	$ 153,732	$ 153,732	$ 109,540
Change in contract assets		44,192
Contract liabilities - current	(90,478)	(90,478)	(84,123)
Change in contract liabilities - current		(6,355)
Contract liabilities - noncurrent	(399)	(399)	0
Change in contract liabilities - noncurrent		(399)
Net contract assets	62,855	62,855	$ 25,417
Change in net contract assets		37,438
Amounts included in contract liability at the beginning of the period	16,900	68,900
Amounts from performance obligations satisfied in prior periods	$ 2,600	$ 5,300