Revenues Contract Asset and Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Contract Assets and Liabilities
Short-term unbilled receivables - included in accounts receivable and unbilled receivables	$ 8,708	$ 9,191
Total contract assets	20,131	25,672
Deferred revenues, net	90,104	81,835
Long-term deferred revenues	10,302	10,582
Total contract liabilities	100,406	92,417
Long-term unbilled receivables - included in other long-term assets
Contract Assets and Liabilities
Other long-term assets	$ 11,423	$ 16,481