Revenue - Changes in contract assets and liabilities (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)
Contract with Customer, Asset and Liability [Abstract]
Increase in net contract assets related to signing incentives	$ 1,273
Decrease in net contract assets and increase in net contract liabilities related to signing incentive amortization	(8,211)
Decrease in net contract liabilities related to signing incentive amortization	(3,990)
Increase in net contract assets related to revenue recognized in advance of billing	2,654
Change in net contract assets due to amounts transferred to receivables	(1,723)
Increase in net contract liabilities primarily relating to cash received from customers	(41,533)
Decrease in net contract assets primarily relating to cash received from customers	(608)
Revenue recognized that was included in the net contract liability balance	40,697
Deferred revenue that was released from the net contract asset balance	$ 2,112