Debt and Credit Agreements (Details)	12 Months Ended								12 Months Ended
	Dec. 31, 2011 USD ($) facility	Dec. 31, 2010 USD ($)	Dec. 31, 2011 Commercial paper USD ($)	Dec. 31, 2010 Commercial paper USD ($)	Dec. 31, 2011 U.S. commercial paper program USD ($)	Dec. 31, 2011 Euro commercial paper program USD ($)	Dec. 31, 2011 Euro commercial paper program EUR (€)	Dec. 31, 2010 Euro commercial paper program USD ($)	Dec. 31, 2011 Multi-year revolving credit facility USD ($) bank	Dec. 31, 2011 Multi-year revolving credit facility Citibank Prime Rate	Dec. 31, 2011 Multi-year revolving credit facility Federal Funds Effective Rate	Dec. 31, 2011 Multi-year revolving credit facility LIBOR	Dec. 31, 2011 Bilateral credit facility USD ($) M
Credit Facilities
Facility Limit	$ 1,404,200,000				$ 550,000,000	$ 259,200,000	€ 200,000,000		$ 570,000,000				$ 25,000,000
Outstanding Balance	39,986,000				39,986,000			26,700,000
Available Credit	1,364,214,000				510,014,000	259,200,000			570,000,000				25,000,000
Aggregate maximum borrowing internal limit under commercial paper and credit facilities	595,000,000
Number of lenders providing credit facility (in banks)									21
Variable basis of interest rates										Citibank Prime Rate	Federal Funds Effective Rate	LIBOR	LIBOR
Facility fee (as a percent)									0.375%
Repayment period after expiration (in months)													12
Short-term borrowings	51,414,000	31,197,000	40,000,000	26,700,000
Weighted-average interest rate for short-term borrowings (as a percent)	1.30%	1.90%
Debt to capital ratio, maximum (as a percent)	60.00%
Minimum net worth	$ 475,000,000
Minimum net worth, number of credit facilities	1
Subsidiary consolidated indebtedness as a percent of consolidated tangible assets, maximum	10.00%