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Income Taxes (Tables)
 9 Months Ended
Sep. 30, 2017
Income Tax Disclosure [Abstract]  
Reconciliation of Total Gross Unrecognized Tax Benefits
(millions)
December 31, 2016
$
63

Tax positions related to current year:
 
Additions
4

Reductions
—

Tax positions related to prior years:
 
Additions
3

Reductions
(8
)
Settlements
(4
)
Lapse in statute of limitations
(2
)
September 30, 2017
$
56