Segment Information - Special Item Charges (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
After-tax Impact [Abstract]
Merger-related transaction costs	$ 46	$ 29	$ 108	$ 29
Charges associated with litigation matters	25		25
Debt extinguishment costs				65
Total Before-tax Impact [Abstract]
Merger-related costs, before tax	49	$ 35	123	35
Charges associated with litigation matters, before tax	40		40
Debt extinguishment costs, before tax			0	100
Total special items	$ 71		$ 133	$ 94