Income Taxes - Unrecognized Tax Benefits (Details) $ in Millions	9 Months Ended
	Sep. 30, 2017 USD ($)
Reconciliation of Unrecognized Tax Benefits [Roll Forward]
Balance at beginning of year	$ 802
Additions due to acquisitions	15
Additions for tax positions of prior years	43
Reductions for tax positions of prior years	(12)
Balance at end of year	$ 866