Debt (Details) (USD $)			3 Months Ended		9 Months Ended
	Sep. 30, 2011	Dec. 31, 2010	Sep. 30, 2011 Unsecured Credit Facility [Member]	Sep. 30, 2010 Unsecured Credit Facility [Member]	Sep. 30, 2011 Unsecured Credit Facility [Member]	Jun. 30, 2011 Unsecured Credit Facility [Member]	May 31, 2011 Unsecured Credit Facility [Member]	Sep. 30, 2011 Overdraft Facilities [Member]
Line of Credit Facility [Line Items]
Term loan portion of credit facility terminated						$ 195,000,000
Maximum borrowing capacity			1,100,000,000		1,100,000,000		900,000,000	48,200,000
Maturity date					June 2016
Number of banks participating in the Facility			18		18
Amount outstanding	567,000,000	197,500,000	567,000,000		567,000,000
Average outstanding borrowings					421,500,000
Short-term borrowings	$ 53,853,000	$ 28,700,000						$ 39,700,000
Pricing on the Facility based on market rates					LIBOR plus 112.5 basis points to LIBOR plus 225.0 basis points
Description of variable rate basis					LIBOR
Basis spread on variable rate (in hundredths)			1.625%		1.625%
Effective interest rate (in hundredths)			1.60%	3.90%
Covenant terms, minimum leverage ratio through September 2013			3.50		3.50
Covenant terms, minimum leverage ratio after September 2013			3.25		3.25
Covenant terms, minimum cash interest coverage ratio			3.00		3.00