Summary of Significant Accounting Policies - Remaining Performance Obligations (Details) $ in Billions	Dec. 31, 2018 USD ($)
Accounting Policies [Abstract]
Revenue, Remaining Performance Obligation, Amount	$ 11.6
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-01-01
Accounting Policies [Abstract]
Revenue, Remaining Performance Obligation, Amount	7.3
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01
Accounting Policies [Abstract]
Revenue, Remaining Performance Obligation, Amount	2.7
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01
Accounting Policies [Abstract]
Revenue, Remaining Performance Obligation, Amount	0.7
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01
Accounting Policies [Abstract]
Revenue, Remaining Performance Obligation, Amount	$ 0.9