Debt and Credit Agreements (Details) (USD $)			3 Months Ended		6 Months Ended
	Jun. 30, 2012	Dec. 31, 2011	Mar. 31, 2012 Amended and Restated Five Year Credit Agreement bank	Mar. 31, 2012 Amended and Restated Five Year Credit Agreement LIBOR	Jun. 30, 2012 Multi-year revolving credit facility	Jun. 30, 2012 US Commercial Paper	Dec. 31, 2011 US Commercial Paper
Credit Facilities
Facility limit			$ 525,000,000
Term of credit agreement			5 years
Number of lenders providing credit facility			14
Available credit			550,000,000
Debt to capital ratio, maximum			60.00%
Subsidiary consolidated indebtedness as a percent of consolidated tangible assets, maximum			10.00%
Total consolidated EBITDA to total consolidated interest charges ratio, numerator, minimum			3
Total consolidated EBITDA to total consolidated interest charges ratio, denominator			1
Variable basis of interest rates				LIBOR
Write-down of deferred financing costs					500,000
Commercial paper outstanding, long-term and short-term combined amount						107,600,000
Commercial paper outstanding, short-term	25,745,000	51,414,000				13,000,000	40,000,000
Commercial paper outstanding, long-term	$ 951,800,000	$ 857,400,000				$ 94,600,000