Revenue Recognition - Additional Information (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Customer relationship period	20 years
Remaining performance obligation	$ 15,500
Contract acquisition costs and fulfillment costs	$ 323
Minimum
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Contract term	1 year
Maximum
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Contract term	7 years
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-04-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Remaining performance obligation, percent	50.00%
Remaining performance obligation, satisfaction period	2 years 9 months
Contract Acquisition and Fulfillment Costs | Minimum
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Amortization period	30 months
Contract Acquisition and Fulfillment Costs | Maximum
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Amortization period	49 months