Income Taxes - Unrecognized Tax Benefits (Details) $ in Millions	6 Months Ended
	Jun. 30, 2018 USD ($)
Reconciliation of Unrecognized Tax Benefits [Roll Forward]
Balance at beginning of year	$ 1,409
Reductions due to acquisitions	(9)
Additions for tax positions of current year	3
Additions for tax positions of prior years	5
Reductions for tax positions of prior years	(69)
Settlements	(52)
Balance at end of year	$ 1,287