Nature of Operations and Basis of Presentation - Narrative (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($) Segment	Dec. 31, 2018 USD ($)	Mar. 31, 2018 USD ($)
Disaggregation of Revenue [Line Items]
Number of reportable segments | Segment	4
Restricted cash	$ 28	$ 32
Contract asset	34	$ 26
Revenue expected to be recognized as of period end	1,600
Amortization period of capitalized contract cost			5 years
Accounting Standards Update 2014-09 | Difference between Revenue Guidance in Effect before and after Topic 606
Disaggregation of Revenue [Line Items]
Unearned revenue	746
Capitalized contract costs	$ 98		$ 81