Debt (Details) - USD ($)		3 Months Ended
	Mar. 07, 2018	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument
Net proceeds of issuance, after the deduction of the underwriting discount and expenses		$ 491,000,000
Basis points added to current Treasury rate used in calculation of alternative redemption price	0.15%
Commercial paper outstanding		$ 0	$ 100,000,000
4.05% Senior notes due March 7, 2048 [Member]
Debt Instrument
Debt, principal amount	$ 500,000,000
Interest rate (percent)	4.05%
Net proceeds of issuance, after the deduction of the underwriting discount and expenses	$ 491,000,000
Percentage of principal amount at which redemption price may be set	100.00%	100.00%