Acquisitions and Dispositions (Schedule of Major Classes of Assets and Liabilities Reported As Discontinued Operation) (Parenthetical) (Detail) $ in Millions	12 Months Ended
Dec. 31, 2018 USD ($)
East Ohio
Business Acquisition [Line Items]
Cash, restricted cash and equivalents of discontinued operations	$ 9
DGP
Business Acquisition [Line Items]
Cash, restricted cash and equivalents of discontinued operations	cash and cash equivalents of less than $1 million.

Amount of cash and cash equivalents, and cash and cash equivalents restricted to withdrawal or usage; attributable to disposal group, including, but not limited to, discontinued operations. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 24
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3521-108585

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=98513485&loc=SL98516268-108586

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3044-108585

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant