Income Taxes (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Income Tax Contingency [Line Items]
Effective tax rate	15.60%	14.50%
IRS Notice proposed assessments for tax years 2005 and 2006			$ 75
Penalties associated with IRS Notice of proposed assessments			$ 52