Revenues - Changes in contract assets and liabilities (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($)
Contract Assets
Increase in net contract assets related to signing incentives additions	$ 22,540
Decrease in net contract assets related to signing incentive amortization	(6,721)
Increase in net contract assets related to revenue recognized in advance of billing	6,005
Change in net contract assets due to amounts transferred to receivables	(2,004)
Changes in net contract assets primarily relating to cash received from customers	(1,396)
Deferred revenue that was released from the net contract asset balance	2,349
Contract Liabilities
Increase in net contract liabilities related to signing incentive amortization	(1,093)
Increase in net contract liabilities primarily relating to cash received from customers	(47,981)
Deferred revenue that was released from net contract liabilities	$ 37,294