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Revenue From Contracts With Customers - Narrative (Details) $ in Millions | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Jun. 30, 2019 USD ($) contract | Jun. 30, 2018 USD ($) | Jun. 30, 2019 USD ($) contract | Jun. 30, 2018 USD ($) | Dec. 31, 2018 USD ($)
Disaggregation of Revenue [Line Items] |  |  |  |  |
Remaining performance obligation | $ 909.0 |  | $ 909.0 |  |
Heavy Civil Construction |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |
Number of contracts (more than) | contract | 150 |  | 150 |  |
Costs and Estimated Earnings in Excess of Billings |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |
Contracts receivable unpaid project contract price | $ 15.7 |  | $ 15.7 |  | $ 9.3
Operating income (Loss) |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |
Estimated construction gain (loss) before tax | $ 3.5 | $ 0.3 | $ 3.3 | $ 1.7 |
Transferred over Time | Product Concentration Risk | Revenue from Contract with Customer |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |
Concentration risk, percentage | 86.00% | 83.00% | 84.00% | 84.00% |
Transferred at Point in Time | Product Concentration Risk | Revenue from Contract with Customer |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |
Concentration risk, percentage | 14.00% | 17.00% | 16.00% | 16.00% |
Minimum |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |
Revenue recognition, contract term |  |  | 12 months |  |
Maximum |  |  |  |  |
Disaggregation of Revenue [Line Items] |  |  |  |  |
Revenue recognition, contract term |  |  | 36 months |  |

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