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Fair Value Measurements - Fair Value of Other Financial Instruments (Details) - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Level Two    
Debt    
Fair value of debt $ 180 $ 244
Market approach valuation technique | Level Two    
Debt    
Fair value of debt 59  
Observable recent pricing of similar instruments valuation technique | Level Three    
Debt    
Fair value of debt $ 121