Revenue - Changes in contract assets and liabilities (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($)
Revenue
Increase in net contract assets related to signing incentives	$ 13,960
Decrease in net contract assets and increase in net contract liabilities related to signing incentive amortization	(13,713)
Decrease in net contract liabilities related to signing incentive amortization	(4,797)
Increase in net contract assets related to revenue recognized in advance of billing	3,481
Change in net contract assets due to amounts transferred to receivables	(2,287)
Increase in net contract liabilities primarily relating to cash received from customers	(72,625)
Decrease in net contract assets primarily relating to cash received from customers	(538)
Revenue recognized that was included in the net contract liability balance	71,789
Deferred revenue that was released from the net contract asset balance	$ 4,380