Commitments and Contingencies - Additional Information (Detail) $ in Millions	Jun. 30, 2018 USD ($)
Gain Contingencies [Line Items]
Purchase commitments, remainder of fiscal year	$ 753
Electricity
Gain Contingencies [Line Items]
Purchase commitments	5
Diesel Fuel
Gain Contingencies [Line Items]
Purchase commitments	$ 22

Minimum amount of purchase arrangement maturing in the remainder of the fiscal year following the latest fiscal year ended. Includes, but is not limited to, recorded and unrecorded purchase obligations, long-term purchase commitments, and short-term purchase commitments.

No definition available.

Name:	us-gaap_PurchaseObligationFutureMinimumPaymentsRemainderOfFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant