Revenue Recognition - Capitalized Contract Costs (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
Capitalized Contract Cost [Roll Forward]
End of period balance	$ 323
Contract Acquisition Costs
Capitalized Contract Cost [Roll Forward]
Beginning of period balance	254
Costs incurred	37
Amortization	(37)
Impairments	0
End of period balance	254
Contract Fulfillment Costs
Capitalized Contract Cost [Roll Forward]
Beginning of period balance	73
Costs incurred	6
Amortization	(10)
Impairments	0
End of period balance	$ 69