Debt (Details) (USD $)	Jun. 30, 2013		Dec. 31, 2012
Debt Instrument [Line Items]
Total debt	$ 799,000,000		$ 1,256,000,000
Less: short-term debt including current maturities	0		457,000,000
Long-term debt	799,000,000		799,000,000
Debt (Textual) [Abstract]
Long-term Debt, Fair Value	800,000,000		900,000,000
5.9% Senior Notes, due 2017 [Member]
Debt Instrument [Line Items]
Total debt	400,000,000	[1]	400,000,000	[1]
Debt (Textual) [Abstract]
Interest rate	5.90%		5.90%
Unamortized debt discount on senior notes	400,000
6.55% Senior Notes, due 2037 [Member]
Debt Instrument [Line Items]
Total debt	399,000,000	[2]	399,000,000	[2]
Debt (Textual) [Abstract]
Interest rate	6.55%		6.55%
Unamortized debt discount on senior notes	1,300,000
Notes Payable [Member]
Debt Instrument [Line Items]
Total debt	$ 0		$ 457,000,000

[1] Interest payments are due semiannually on April 15 and October 15, and, as of June 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 June 30, 2013, the unamortized debt discount is $1.3 million.