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Summary of Significant Accounting Policies - Revenue Recognition (Detail) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2017 | Sep. 30, 2016 | Sep. 30, 2017 | Sep. 30, 2016 | Dec. 31, 2016
Revenue Recognition [Line Items] |  |  |  |  |
Percent increase (decrease) in operating results due to change in contract estimates |  | 6.30% |  |  |
Change orders and/or claims | $ 115.5 |  | $ 115.5 |  | $ 137.8
Maximum |  |  |  |  |
Revenue Recognition [Line Items] |  |  |  |  |
Percent aggregate change in contract estimates impact on operating results is less than this percentage | 5.00% |  | 5.00% | 5.00% |
Alaska Power Plant Construction Project |  |  |  |  |
Revenue Recognition [Line Items] |  |  |  |  |
Losses on contracts |  | $ 3.0 |  | $ 54.8 |

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