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Fair Value Measurements - Additional Information (Detail)
$ in Millions, € in Billions, ¥ in Billions
3 Months Ended 12 Months Ended
Dec. 31, 2017
USD ($)
Sep. 30, 2017
USD ($)
Dec. 31, 2017
EUR (€)
Dec. 31, 2017
USD ($)
Dec. 31, 2017
JPY (¥)
Sep. 30, 2017
EUR (€)
Sep. 30, 2017
USD ($)
Sep. 30, 2017
JPY (¥)
Derivative [Line Items]                
Unrealized gains on marketable common stock $ 14 $ 10            
Unrealized losses on marketable common stock $ 10 $ 6            
Fair value of long term debt       $ 11,500     $ 12,700  
Fair Value, Inputs, Level 1 [Member]                
Derivative [Line Items]                
Fair value of long term debt       9,300     8,600  
Fair Value, Inputs, Level 2 [Member]                
Derivative [Line Items]                
Fair value of long term debt       $ 2,200     $ 4,100  
Foreign Currency Denominated Debt [Member]                
Derivative [Line Items]                
Notional amount | ¥         ¥ 35     ¥ 35
Foreign Currency Denominated Debt [Member] | One billion euro net investment hedge [Member]                
Derivative [Line Items]                
Notional amount | €     € 1     € 1