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Summary of Significant Accounting Policies - Goodwill (Details) $ in Thousands | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2019 USD ($) division | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($)
Goodwill And Intangible Assets [Line Items] |  |  |
Number of internal divisions | division | 2 |  |
Decrease in fair value of reporting units considered for impairment calculation | 10.00% |  |
Goodwill | $ 1,927,028 | $ 1,899,879 | $ 1,868,600
Intangible assets | 276,070 | $ 280,180 |
Oil and Gas Infrastructure Services Division |  |  |
Goodwill And Intangible Assets [Line Items] |  |  |
Goodwill | 48,500 |  |
Intangible assets | $ 10,700 |  |

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