Long-Term Debt (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Long-Term Debt
Long-term debt	$ 3,116	$ 2,595
2016 Notes [Member]
Long-Term Debt
Senior Long Term Notes	500	500
2017 Notes [Member]
Long-Term Debt
Senior Long Term Notes	500
2019 Notes [Member]
Long-Term Debt
Senior Long Term Notes	866	845
2020 Notes [Member]
Long-Term Debt
Senior Long Term Notes	750	750
2040 Notes [Member]
Long-Term Debt
Senior Long Term Notes	250	250
2041 Notes [Member]
Long-Term Debt
Senior Long Term Notes	$ 250	$ 250