Revenue Recognition - Capitalized Contract Costs (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018
Contract Acquisition Costs
Capitalized Contract Cost [Roll Forward]
Beginning of period balance	$ 286	$ 268
Costs incurred	53	152
Amortization	(44)	(125)
End of period balance	295	295
Contract Fulfillment Costs
Capitalized Contract Cost [Roll Forward]
Beginning of period balance	161	133
Costs incurred	46	105
Amortization	(34)	(65)
End of period balance	$ 173	$ 173