Revenue Recognition - Contract Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Acquisition Costs
Capitalized Contract Cost [Line Items]
Beginning of period balance	$ 90	$ 91
Costs incurred	62
Amortization	(63)
End of period balance	90
Fulfillment Costs
Capitalized Contract Cost [Line Items]
Beginning of period balance	57	$ 61
Costs incurred	27
Amortization	(31)
End of period balance	$ 57