Unrecognized Tax Benefits Reconciliation (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Balance at January 1	$ 11	$ 11
Balance at December 31	$ 11	$ 11

Amount of unrecognized tax benefits.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 15A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=84230637&loc=SL6600010-109319

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