Revenue - Changes in contract assets and liabilities (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($)
Contract Assets
Increase in net contract assets related to signing incentives	$ 18,366
Decrease in net contract assets related to signing incentive amortization	(19,980)
Increase in net contract assets related to revenue recognized in advance of billing	3,900
Change in net contract assets due to amounts transferred to receivables	(3,694)
Changes in net contract assets primarily relating to cash received from customers	(3,925)
Deferred revenue that was released from the net contract asset balance	6,308
Contract Liabilities
Decrease in net contract liabilities related to signing incentives	57
Increase in net contract liabilities related to signing incentive amortization	(5,986)
Decrease in net contract liabilities related to revenue recognized in advance of billing	1,584
Change in net contract liabillities due to amunts transferred to receivables	(2,612)
Increase in net contract liabilities primarily relating to cash received from customers	(111,778)
Deferred revenue that was released from net contract liabilities	$ 113,520