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Contract Balances (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Jan. 01, 2018
Change in Contract with Customer, Asset and Liability [Abstract]        
Revenue recognized that was included in the contract liability balance at the beginning of the period $ (15,414) $ (13,781)    
Contract with Customer, Asset and Liability [Abstract]        
Contract assets balance 11,443   $ 9,330 $ 11,932
Contract liabilities - current 39,733   36,461 48,268
Contract liabilities - non-current $ 9,731   $ 9,382 $ 9,916