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Fair Value Measurements - Fair Values - Reported (Detail) - USD ($)
$ in Billions
Sep. 30, 2019
Dec. 31, 2018
Fair Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt $ 30.2 $ 26.5
Carrying Value    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt $ 28.3 $ 27.0