Goodwill, Other Intangibles, and Property and Equipment - Property and Equipment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Cost	$ 7,958	$ 4,613
Accumulated amortization	3,396	3,050
Net carrying value	$ 4,562	1,563
Capital lease agreements, term length	4 years
Internal-use software [Member]
Property Plant And Equipment [Line Items]
Cost	$ 5,694	2,991
Accumulated amortization	2,415	2,184
Net carrying value	3,279	807
Total other property and equipment [Member]
Property Plant And Equipment [Line Items]
Cost	2,264	1,622
Accumulated amortization	981	866
Net carrying value	1,283	756
Assets recorded under capital leases [Member]
Property Plant And Equipment [Line Items]
Cost	56	49
Accumulated amortization	4	31
Net carrying value	52	18
Other property and equipment not recorded under capital leases [Member]
Property Plant And Equipment [Line Items]
Cost	2,208	1,573
Accumulated amortization	977	835
Net carrying value	$ 1,231	$ 738