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Revenue Recognition - Receivables, Contract Assets and Liabilities from Contracts with Customer (Details) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Apr. 01, 2018
Dec. 31, 2017
Revenue from Contract with Customer [Abstract]        
Contract Assets $ 51,332 $ 48,786 $ 48,786 $ 45,877
Contract Liabilities $ (14,757) $ (18,533) $ (18,533) $ (17,736)