Income Taxes - Unrecognized Tax Benefits (Details) $ in Millions	3 Months Ended
	Mar. 31, 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	1
Additions for tax positions of prior years	1
Reductions for tax positions of prior years	(84)
Settlements	(45)
Balance at end of year	$ 1,273