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	3 Months Ended
	Mar. 31, 2018 USD ($)
Remaining Performance Obligations
Remaining performance obligations	$ 1,080
Expected timing of performance obligations	12 months
Expected percentage of remaining performance obligations	85.00%