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Income Taxes - Reconciliation of Changes in Federal, State and Foreign Unrecognized Tax Position Balances (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Income Tax Uncertainties [Abstract]      
Balance at beginning of period $ 243 $ 267 $ 264
Additions (reductions) for tax positions taken in prior years 57 (17) 31
Additions for tax positions taken in the current year 12 13 4
Exam resolutions (6) (17) (22)
Statute expirations (4) (3) (10)
Balance at end of period $ 302 $ 243 $ 267