Revenue - Opening and Closing Balances (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Revenue from Contract with Customer [Abstract]
Receivables	$ 645,468	$ 576,313	$ 576,313
Increase (decrease) in receivables	69,155
Contract asset, current	9,580	9,002
Increase (decrease) in contract asset, current	578
Contract asset, non-current	16,790	16,186
Increase (decrease) in contract asset, non-current	604
Deferred revenue, current	71,571	71,085
Increase (decrease) in deferred revenue, current	486
Deferred revenue, non-current	51,098	$ 53,101
Increase (decrease) in deferred revenue, non-current	$ (2,003)