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Financial Instruments, Derivatives and Hedging Activities - Interest rate hedging (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Financial instruments, derivatives and hedging activities    
Accumulated gains (losses) from derivative instruments, net of tax effect $ (1,160) $ (1,154)
4.625% senior notes due November 1, 2020    
Financial instruments, derivatives and hedging activities    
Debt, face amount 400  
Treasury Lock    
Financial instruments, derivatives and hedging activities    
Derivative notional amount 0 0
Cash Flow Hedging | Treasury Lock    
Financial instruments, derivatives and hedging activities    
Accumulated gains (losses) from derivative instruments, net of tax effect (2) (2)
Tax effect on gain (loss) on derivative instruments (1) (1)
Fair Value Hedging | Interest Rate Swap    
Financial instruments, derivatives and hedging activities    
Increase (decrease) in fair value of instrument   $ (1)
Fair Value Hedging | Interest Rate Swap | Maximum    
Financial instruments, derivatives and hedging activities    
Increase (decrease) in fair value of instrument 1  
Fair Value Hedging | Interest Rate Swap | 4.625% senior notes due November 1, 2020    
Financial instruments, derivatives and hedging activities    
Debt, face amount $ 200  
Debt, fixed interest rate (as a percent) 4.625%  
Debt, floating rate of interest basis six-month US dollar LIBOR