Debt (Details)					6 Months Ended
	Apr. 16, 2018 USD ($)	Jun. 27, 2017 USD ($)	Jun. 21, 2017 USD ($)	May 29, 2015 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Mar. 28, 2013 USD ($)
Debt
Retirement of notes					$ 114,000,000	$ 535,000,000
Proceeds from issuance of debt			$ 599,000,000
Interest expense, debt						$ 107,000,000
Document Period End Date					Jun. 30, 2018
Estimate of Fair Value Measurement
Debt
Estimated market value of long-term and short-term debt					$ 3,000,000,000		$ 3,300,000,000
Carrying Value Reported Value Measurement
Debt
Aggregate carrying value of long-term and short-term debt					3,000,000,000		$ 3,100,000,000
Credit Agreement dated March 28, 2013
Debt
Borrowing capacity, maximum								$ 1,250,000,000
Credit agreement dated May 29, 2015 and August 28, 2015, as amended
Debt
Borrowing capacity, maximum				$ 750,000,000
Increase in maximum borrowing capacity				$ 375,000,000
Maximum net leverage ratio				4
Minimum interest coverage ratio				2.5
Borrowings outstanding					0
Credit agreement dated May 29, 2015 and August 28, 2015, as amended | Federal funds effective rate
Debt
Interest rate, basis spread (as a percent)				0.50%
Credit agreement dated May 29, 2015 and August 28, 2015, as amended | Libor rate
Debt
Interest rate, basis spread (as a percent)				1.00%
Credit agreement dated May 29, 2015 and August 28, 2015, as amended | Revolver
Debt
Borrowing capacity, maximum					500,000,000
Credit agreement dated May 29, 2015 and August 28, 2015, as amended | Swingline loans
Debt
Borrowing capacity, maximum					75,000,000
Credit agreement dated May 29, 2015 and August 28, 2015, as amended | Letters of credit
Debt
Borrowing capacity, maximum					100,000,000
Outstanding and unused Letters of Credit					$ 0
6.625% Notes and Debentures Due April 15, 2018
Debt
Retirement of notes	$ 114,000,000
Debt instrument, interest rate	6.625%
3.5% Notes and Debentures Due November 15, 2027
Debt
Debt instrument, interest rate			3.50%
Debt issued			$ 300,000,000
4.5% Notes and Debentures Due May 15, 2047
Debt
Debt instrument, interest rate			4.50%
Debt issued			$ 300,000,000
7.125% Notes and Debentures due March 15, 2020
Debt
Retirement of notes		$ 299,000,000
Debt instrument, interest rate		7.125%
5.95% Notes and Debentures Due March 15, 2022
Debt
Retirement of notes		$ 74,000,000
Debt instrument, interest rate		5.95%
7.75% Notes And Debentures due 2029
Debt
Retirement of notes		$ 62,000,000
Debt instrument, interest rate		7.75%
6.5% Notes and Debentures due 2032
Debt
Retirement of notes		$ 100,000,000
Debt instrument, interest rate		6.50%