Goodwill, Other Intangibles, and Property and Equipment - Property and Equipment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Property Plant And Equipment [Line Items]
Cost	$ 4,016	$ 3,931
Accumulated amortization	2,482	2,429
Net carrying value	$ 1,534	1,502
Capital lease agreements, term length	4 years
Internal-use software [Member]
Property Plant And Equipment [Line Items]
Cost	$ 2,442	2,191
Accumulated amortization	1,708	1,467
Net carrying value	734	724
Other property and equipment [Member]
Property Plant And Equipment [Line Items]
Cost	1,574	1,740
Accumulated amortization	774	962
Net carrying value	800	778
Assets Recorded under Capital Leases [Member]
Property Plant And Equipment [Line Items]
Cost	90	84
Accumulated amortization	44	36
Net carrying value	$ 46	$ 48