Debt (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Debt Instrument [Line Items]
Total debt	$ 799		$ 1,256
Less: short-term debt including current maturities	0		457
Long-term debt	799		799
5.9% Senior Notes, due 2017
Debt Instrument [Line Items]
Total debt	400	[1]	400	[1]
Interest rate	5.90%		5.90%
Unamortized debt discount on senior notes	0.4
6.55% Senior Notes, due 2037
Debt Instrument [Line Items]
Total debt	399	[2]	399	[2]
Interest rate	6.55%		6.55%
Unamortized debt discount on senior notes	1.3
Commercial paper
Debt Instrument [Line Items]
Total debt	$ 0		$ 457

[1] Interest payments are due semiannually on April 15 and October 15, and, as of September 30, 2013, the unamortized debt discount is $0.4 million.

[2] Interest payments are due semiannually on May 15 and November 15, and, as of September 30, 2013, the unamortized debt discount is $1.3 million.