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Revenue Recognition - Performance Obligations (Details)
$ in Millions
Mar. 31, 2019
USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-04-01  
Revenue from Contract with Customer [Abstract]  
Estimated future fixed fee performance obligations $ 500
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]  
Revenue remaining performance obligation period 9 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01  
Revenue from Contract with Customer [Abstract]  
Estimated future fixed fee performance obligations $ 500
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]  
Revenue remaining performance obligation period 1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01  
Revenue from Contract with Customer [Abstract]  
Estimated future fixed fee performance obligations $ 535
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]  
Revenue remaining performance obligation period 1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01  
Revenue from Contract with Customer [Abstract]  
Estimated future fixed fee performance obligations $ 284
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]  
Revenue remaining performance obligation period 1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2023-01-01  
Revenue from Contract with Customer [Abstract]  
Estimated future fixed fee performance obligations $ 29
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]  
Revenue remaining performance obligation period 1 year