Contingencies - Additional Information (Detail) $ in Millions	Dec. 31, 2017 USD ($)
Commitments And Contingencies Disclosure [Abstract]
Accrual for disclosed loss contingencies, current, ending balance	$ 370
Amount not accrued for estimated maximum losses for disclosed loss contingencies	$ 1,300

The difference between the maximum amount of estimated loss and the amount recorded as of the balance sheet date.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 05
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6395354&loc=d3e12631-108344

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