Summary of Accounting Policies - Risk Management Activities (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Foreign Exchange Forward
Derivative [Line Items]
Fair value of derivative liability	$ 1	$ 1

Aggregate fair value of all derivative liabilities designated as hedging instruments. Includes instruments designated as cash flow hedges, fair value hedges, and hedges of net investments in foreign operations.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13433-108611

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