Goodwill, Other Intangibles, and Property and Equipment - Depreciation and Amortization (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Depreciation And Amortization By Type [Line Items]
Depreciation and amortization	$ 695	$ 566	$ 610
Internal-use software [Member]
Depreciation And Amortization By Type [Line Items]
Depreciation and amortization	323	298	303
Other property and equipment [Member]
Depreciation And Amortization By Type [Line Items]
Depreciation and amortization	146	153	158
Assets recorded under capital leases [Member]
Depreciation And Amortization By Type [Line Items]
Amortization on assets under capital leases	9	14	20
Value of business acquired [Member]
Depreciation And Amortization By Type [Line Items]
Depreciation and amortization	16	18	20
Other intangibles [Member]
Depreciation And Amortization By Type [Line Items]
Depreciation and amortization	$ 210	$ 97	$ 129