Valuation Accounts (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Allowance for doubtful accounts
Balance at beginning of period	$ 42	$ 32	$ 38
Additions charged to costs and expenses	17	10	(6)
Other additions	0	0	0
Deductions	5	0	0
Balance at end of period	$ 54	$ 42	$ 32