Summary of Accounting Policies - Internal Use Software Assets/Accounts Payable (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Finite-Lived Intangible Assets [Line Items]
Capitalized software development costs, net of amortization	$ 66	$ 58
Capitalized Computer Software, Amortization	$ 12	13	$ 15
Minimum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite useful life of intangible assets, Maximum	3 years
Minimum [Member] | Software Development [Member]
Finite-Lived Intangible Assets [Line Items]
Finite useful life of intangible assets, Maximum	3 years
Maximum [Member]
Finite-Lived Intangible Assets [Line Items]
Finite useful life of intangible assets, Maximum	50 years
Maximum [Member] | Software Development [Member]
Finite-Lived Intangible Assets [Line Items]
Finite useful life of intangible assets, Maximum	12 years
Accounts Payable [Member]
Finite-Lived Intangible Assets [Line Items]
Accounts payable	$ 99	93
Accrued compensation	$ 27	$ 17