Revenue Recognition - Capitalized Contract Costs (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Contract Acquisition Costs
Capitalized Contract Cost [Roll Forward]
Beginning of period balance	$ 322	$ 268
Costs incurred	57	52
Amortization	(50)	(39)
End of period balance	329	281
Contract Fulfillment Costs
Capitalized Contract Cost [Roll Forward]
Beginning of period balance	187	133
Costs incurred	34	29
Amortization	(23)	(12)
End of period balance	$ 198	$ 150