Revenue Recognition - Capitalized Contract Costs (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018
Contract Acquisition Costs
Capitalized Contract Cost [Roll Forward]
Beginning of period balance	$ 280	$ 268
Costs incurred	47	99
Amortization	(41)	(81)
End of period balance	286	286
Contract Fulfillment Costs
Capitalized Contract Cost [Roll Forward]
Beginning of period balance	105	88
Costs incurred	29	59
Amortization	(18)	(31)
End of period balance	$ 116	$ 116