Revenue - Remaining Performance Obligations to be Recognized as Revenue (Details) - USD ($) $ in Billions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-01-01
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue, remaining performance obligations	$ 69.5	$ 63.2
Revenue, remaining performance obligation, percentage recognized		40.00%
Revenue, remaining performance obligation, expected timing of satisfaction, period		1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-01-01
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue, remaining performance obligation, percentage recognized	50.00%	40.00%
Revenue, remaining performance obligation, expected timing of satisfaction, period	1 year	2 years
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue, remaining performance obligation, percentage recognized	30.00%
Revenue, remaining performance obligation, expected timing of satisfaction, period	2 years