Debt, Description of long-term debt (Details) - Senior Notes due 2055, $1,000 million, 4.625% ($16 million of discount) [Member] $ in Millions	9 Months Ended
	Sep. 30, 2015 USD ($)
Description of long-term debt changes [Line Items]
Issuance date	March 2015
Face amount	$ 1,000
Interest rate (in hundredths)	4.625%
Maturity year	2055
Discounted price at which long-term debt was issued (in hundredths)	98.353%
Description of debt redemption terms	The notes may be redeemed before the date that is six months prior to the scheduled maturity date at a redemption price equal to the greater of 100% of the principal amount of the notes redeemed and the sum of the present values of the remaining scheduled payments of principal and interest (discounted at the applicable Treasury Yield plus 35 basis points) on the notes to be redeemed. The notes may also be redeemed on or after the date that is six months prior to the final maturity date of the notes at a redemption price equal to 100% of the principal amount of the notes redeemed plus accrued and unpaid interest.