Debt (Details) - USD ($)			9 Months Ended
	Jun. 02, 2019	Mar. 04, 2019	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Debt Instrument
Net proceeds of issuance, after the deduction of the underwriting discount and expenses			$ 492,000,000	$ 591,000,000
Debt, principal amount repaid			500,000,000	$ 600,000,000
Commercial paper outstanding			$ 100,000,000		$ 100,000,000
Senior Notes [Member] | 4.10% Senior notes due March 4, 2049 [Member]
Debt Instrument
Debt, principal amount		$ 500,000,000
Interest rate (percent)		4.10%
Net proceeds of issuance, after the deduction of the underwriting discount and expenses		$ 492,000,000
Percentage of principal amount at which redemption price may be set		100.00%
Basis points added to current Treasury rate used in calculation of alternative redemption price		0.20%
Senior Notes [Member] | 5.90% Senior notes due June 2, 2019 [Member]
Debt Instrument
Interest rate (percent)	5.90%
Debt, principal amount repaid	$ 500,000,000