Goodwill, Other Intangibles, and Property and Equipment - Depreciation and Amortization (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Depreciation And Amortization By Type [Line Items]
Depreciation and amortization	$ 566	$ 610	$ 585
Bargain purchase gain			23
Internal-use software [Member]
Depreciation And Amortization By Type [Line Items]
Depreciation and amortization	298	303	288
Other property and equipment [Member]
Depreciation And Amortization By Type [Line Items]
Depreciation and amortization	153	158	160
Assets recorded under capital leases [Member]
Depreciation And Amortization By Type [Line Items]
Amortization on assets under capital leases	14	20	22
Value of business acquired [Member]
Depreciation And Amortization By Type [Line Items]
Depreciation and amortization	18	20	18
Other intangibles [Member]
Depreciation And Amortization By Type [Line Items]
Depreciation and amortization	$ 97	$ 129	119
Bargain purchase gain			$ 23