Debt (details) - USD ($) $ in Millions	1 Months Ended	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2015	Aug. 25, 2015
Debt Instrument
Net proceeds of issuance, after original issuance discount and deduction of underwriting expenses and commissions and other expenses		$ 392
4.30% Senior notes due August 25, 2045 [Member]
Debt Instrument
Debt, principal amount			$ 400
Interest rate (percent)			4.30%
Net proceeds of issuance, after original issuance discount and deduction of underwriting expenses and commissions and other expenses	$ 392
Debt instrument, redemption option, description	Prior to February 25, 2045, the senior notes may be redeemed, in whole or in part, at the Company's option, at any time or from time to time, at a redemption price equal to the greater of (a) 100% of the principal amount of any senior notes to be redeemed or (b) the sum of the present values of the remaining scheduled payments of principal and interest on any senior notes to be redeemed (exclusive of interest accrued to the date of redemption) discounted to the date of redemption on a semi-annual basis (assuming a 360-day year consisting of twelve 30-day months) at the then current rate of a treasury security having a maturity comparable to the remaining term of these senior notes, plus 25 basis points. On or after February 25, 2045, the senior notes may be redeemed, in whole or in part, at the Company's option, at any time or from time to time, at a redemption price equal to 100% of the principal amount of any senior notes to be redeemed.
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	0.25%