Summary of Significant Accounting Policies - Remaining Performance Obligations (Details) - Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-04-01 $ in Millions	6 Months Ended
	Jun. 30, 2018 USD ($)
Remaining Performance Obligations
Remaining performance obligations	$ 1,230
Expected percentage of remaining performance obligations	85.00%
Percentage of remaining performance obligation expected to be recognized over next 12 months	12 months