Asset Retirement Obligations - Additional Information (Detail) $ in Millions |
12 Months Ended |
---|---|
Dec. 31, 2016
USD ($)
| |
Asset Retirement Obligation Disclosure [Abstract] | |
Financial assurance in form of surety bond | $ 28.2 |
Unfunded standby letter of credit | 8.8 |
Asset retirement obligation liabilities expected, present value | $ 26.9 |