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Income Taxes - Schedule of Changes in Valuation Allowance (Detail) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Income Tax Disclosure [Abstract] |  |  |  |
Balance at beginning of year |  | $ 1,940 | $ 1,291 | $ 1,151
Increase to allowance | $ 1,267 | 831 | 772 | 180
Release of allowance |  | (246) | (209) | (42)
Acquisitions and divestitures (F) |  | (1) | (1) | 29
Tax apportionment, tax rate and tax law changes |  | (24) | 106 | (15)
Foreign currency translation |  | 84 | (19) | (12)
Balance at end of year | $ 1,940 | $ 2,584 | $ 1,940 | $ 1,291

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- Definition Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32537-109319 + Details
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- Definition Amount of increase (decrease) in the valuation allowance for a specified deferred tax asset. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 45 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=84176650&loc=d3e32123-109318 + Details
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