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Revenue (Customer Liability) (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Revenue from Contract with Customer [Abstract]      
Contract liability $ 9,351   $ 7,786
Contract with Customer, Liability, Revenue Recognized $ 1,800 $ 1,500