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Long-Term Debt - Narrative (Details) - USD ($)
3 Months Ended 6 Months Ended
Mar. 31, 2017
Jun. 30, 2017
Debt Instrument [Line Items]    
Contributions to company's qualified defined benefit pension plans   $ 3,200,000,000
Value of common stock from treasury contributed to qualified defined benefit pension plans $ 350,000,000  
Unsecured Debt    
Debt Instrument [Line Items]    
Debt instrument, face amount 2,000,000,000.0  
Unsecured Debt | 2.875% Notes due 2020    
Debt Instrument [Line Items]    
Debt instrument, face amount $ 1,000,000,000.0  
Debt instrument, stated interest rate (percent) 2.875%  
Unsecured Debt | 3.625% Notes due 2022    
Debt Instrument [Line Items]    
Debt instrument, face amount $ 1,000,000,000.0  
Debt instrument, stated interest rate (percent) 3.625%