Summary of Significant Accounting Policies - Remaining Performance Obligations (Details) - Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-04-01 $ in Millions	Sep. 30, 2019 USD ($)
Remaining Performance Obligations
Remaining performance obligations	$ 1,610
Expected timing of performance obligations	12 months
Minimum
Remaining Performance Obligations
Expected percentage of remaining performance obligations	80.00%
Maximum
Remaining Performance Obligations
Expected percentage of remaining performance obligations	85.00%