Commitments and Contingencies - Additional Information (Detail) $ in Millions	Sep. 30, 2017 USD ($)
Gain Contingencies [Line Items]
Purchase commitments	$ 813
Purchase commitments due in remainder of 2017	794
Purchase commitments due in 2018	19
Electricity [Member]
Gain Contingencies [Line Items]
Purchase commitments	5
Diesel Fuel [Member]
Gain Contingencies [Line Items]
Purchase commitments	51
Purchase commitments due in remainder of 2017	18
Purchase commitments due in 2018	$ 33