Debt (Details) (USD $)	9 Months Ended								9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010	Jul. 14, 2011 Commercial Paper [Member]	Sep. 30, 2011 National [Member] Notes Payable [Member]	Sep. 30, 2011 National [Member] Notes Payable [Member] Notes due 2012 at 6.15% (assumed with National acquisition)	Sep. 30, 2011 National [Member] Notes Payable [Member] Notes due 2015 at 3.95% (assumed with National acquisition)	Sep. 30, 2011 National [Member] Notes Payable [Member] Notes due 2017 at 6.60% (assumed with National acquisition)	Sep. 30, 2011 Notes Payable [Member]	May 23, 2011 Notes Payable [Member]	Sep. 30, 2011 Notes Payable [Member] Floating-rate notes due 2013 (swapped to a 0.922% fixed rate)	Sep. 30, 2011 Notes Payable [Member] Floating-rate notes due 2013 (swapped to a 0.922% fixed rate) Interest Rate Swap [Member] Cash Flow Hedges [Member]	Sep. 30, 2011 Notes Payable [Member] Notes due 2013 at 0.875%	Sep. 30, 2011 Notes Payable [Member] Notes due 2014 at 1.375%	Sep. 30, 2011 Notes Payable [Member] Notes due 2016 at 2.375%
Short-term Debt
Commercial paper issued				$ 1,200,000,000						$ 3,500,000,000
Commercial paper outstanding	1,200,000,000
Aggregated principal amount of fixed and floating rate long-term debt				1,200,000,000						3,500,000,000
Proceeds from Issuance of Long-term Debt									3,497,000,000
Payments of Debt Issuance Costs	12,000,000	0							12,000,000
Long-term debt, effective fixed-rate interest rate												0.922%
Long-term debt, stated interest rate						6.15%	3.95%	6.60%			0.922%		0.875%	1.375%	2.375%
Long-term Debt
Long-term Debt, Gross	4,500,000,000				1,000,000,000	375,000,000	250,000,000	375,000,000			1,000,000,000		500,000,000	1,000,000,000	1,000,000,000
Add net unamortized premium (assumed with National acquisition)	101,000,000
Less current portion of long-term debt	(386,000,000)
Total long-term debt	4,215,000,000		0
Long-term Debt, Fair Value					$ 1,105,000,000