Goodwill, Other Intangibles, and Property and Equipment - Property and Equipment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Property Plant And Equipment [Line Items]
Cost	$ 4,613	$ 4,364
Accumulated amortization	3,050	2,828
Net carrying value	$ 1,563	1,536
Capital lease agreements, term length	4 years
Internal-use software [Member]
Property Plant And Equipment [Line Items]
Cost	$ 2,991	2,766
Accumulated amortization	2,184	1,997
Net carrying value	807	769
Total other property and equipment [Member]
Property Plant And Equipment [Line Items]
Cost	1,622	1,598
Accumulated amortization	866	831
Net carrying value	756	767
Assets recorded under capital leases [Member]
Property Plant And Equipment [Line Items]
Cost	49	87
Accumulated amortization	31	49
Net carrying value	18	38
Other property and equipment not recorded under capital leases [Member]
Property Plant And Equipment [Line Items]
Cost	1,573	1,511
Accumulated amortization	835	782
Net carrying value	$ 738	$ 729