Revenue Recognition - Capitalized Contract Costs (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($)
Contract Acquisition Costs
Capitalized Contract Cost [Roll Forward]
Beginning of period balance	$ 268
Costs incurred	226
Amortization	(172)
End of period balance	322
Contract Fulfillment Costs
Capitalized Contract Cost [Roll Forward]
Beginning of period balance	133
Costs incurred	146
Amortization	(92)
End of period balance	$ 187