Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions |
12 Months Ended | |
|---|---|---|
Dec. 31, 2017 |
Dec. 31, 2016 |
|
| Commitments And Contingencies Disclosure [Abstract] | ||
| Percentage of agreement indemnification | 100.00% | |
| Outstanding performance and surety bonds | $ 178.4 | $ 123.5 |