Goodwill, Other Intangibles, and Property and Equipment - Depreciation and Amortization (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015		Dec. 31, 2014
Depreciation And Amortization By Type [Line Items]
Depreciation And Amortization		$ 610	$ 585		$ 588
Internal-use software [Member]
Depreciation And Amortization By Type [Line Items]
Depreciation And Amortization		303	288		260
Other property and equipment [Member]
Depreciation And Amortization By Type [Line Items]
Depreciation And Amortization	[1]	158	160		153
Assets recorded under capital leases [Member]
Depreciation And Amortization By Type [Line Items]
Depreciation And Amortization		20	22		20
Value of business acquired [Member]
Depreciation And Amortization By Type [Line Items]
Depreciation And Amortization		20	18		12
Other intangibles [Member]
Depreciation And Amortization By Type [Line Items]
Depreciation And Amortization		$ 129	119	[2]	$ 163
Bargain purchase gain			$ 23

[1] Other property and equipment includes amortization on assets recorded under capital leases of $20 million in 2016, $22 million in 2015 and $20 million in 2014.

[2] Includes the one-time $23 million benefit of a 2015 acquisition in which the fair value of acquired net assets exceeded the purchase price.