Financial Instruments - Carrying and Estimated Fair Value (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Carrying And Estimated Fair Value [Line Items]
Long-term debt (including current maturities)	$ 1,381	$ 1,358
Equity Swap
Equity swap agreement and foreign currency forward contracts:
Asset derivative contracts	2	4
Net Carrying Amount
Carrying And Estimated Fair Value [Line Items]
Long-term debt (including current maturities)	1,385	1,361
Net Carrying Amount | Equity swap agreement and foreign currency forward contracts
Equity swap agreement and foreign currency forward contracts:
Asset derivative contracts	2	4
Fair Value
Carrying And Estimated Fair Value [Line Items]
Long-term debt (including current maturities)	1,335	1,398
Fair Value | Equity swap agreement and foreign currency forward contracts
Equity swap agreement and foreign currency forward contracts:
Asset derivative contracts	$ 2	$ 4