Nature of Operations and Basis of Presentation - Narrative (Details) $ in Millions	9 Months Ended
	Sep. 30, 2019 USD ($) Segment	Dec. 31, 2018 USD ($)	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Number of reportable segments | Segment	4
Restricted cash	$ 23	$ 41
Contract asset	49	26
Revenue expected to be recognized as of period end	1,500
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	1,500
Capitalized contract costs	$ 96	$ 101