Summary of Significant Accounting Policies - Revenue Recognition, Remaining Performance Obligation (Details) $ in Millions	Sep. 30, 2018 USD ($)
Accounting Policies [Abstract]
Remaining performance obligation	$ 124
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-10-01
Accounting Policies [Abstract]
Remaining performance obligation	$ 87
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Expected timing of satisfaction	2 years
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-10-01
Accounting Policies [Abstract]
Remaining performance obligation	$ 37
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Expected timing of satisfaction