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Revenue - Contract Assets and Liabilities (Details) - USD ($) $ in Thousands | 3 Months Ended |
--- | --- | ---
Mar. 31, 2019 | Mar. 31, 2018 | Dec. 31, 2018
Capitalized Contract Cost [Line Items] |  |  |
Costs in excess of billings | $ 25,519 |  | $ 22,634
Amounts included in billings in excess of cost at the beginning of the period | 9,697 | $ 8,340 |
Amounts included in unearned revenue at the beginning of the period | 4,661 | $ 1,836 |
Billings in excess of cost |  |  |
Capitalized Contract Cost [Line Items] |  |  |
Contract with customer liability | 18,259 |  | 17,857
Unearned revenue |  |  |
Capitalized Contract Cost [Line Items] |  |  |
Contract with customer liability | $ 12,917 |  | $ 12,028

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