Discontinued Operation (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended			1 Months Ended	12 Months Ended			12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Feb. 28, 2007 Traffic Control Business [Member]	Dec. 31, 2010 Traffic Control Business [Member]	Dec. 31, 2009 Traffic Control Business [Member]	Dec. 31, 2011 Traffic Control Business [Member]	Dec. 31, 2011 Maximum [Member] Traffic Control Business [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net proceeds from sale of discontinued operations				$ 66
Liability for Unpaid Claims and Claims Adjustment Expense, Gross					12		8
Loss from discontinued operation, net of taxes	$ 0	$ 4	$ 2		$ 4	$ 2		$ 1