Debt (Narrative) (Details) (USD $)	12 Months Ended			0 Months Ended		12 Months Ended		0 Months Ended	12 Months Ended		0 Months Ended	12 Months Ended				0 Months Ended		12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Jul. 21, 2003 4.375% Senior Notes [Member]	Jul. 21, 2003 5.75% Senior Notes [Member]	Dec. 31, 2011 5.75% Senior Notes [Member]	Dec. 31, 2010 5.75% Senior Notes [Member]	Mar. 25, 2008 6.50% Senior Notes [Member]	Dec. 31, 2011 6.50% Senior Notes [Member]	Dec. 31, 2010 6.50% Senior Notes [Member]	Oct. 11, 2011 Old Senior Credit Facility [Member]	Dec. 31, 2011 New Senior Credit Facility [Member]	Oct. 11, 2011 New Senior Credit Facility [Member] Term Loan Facility [Member]	Oct. 11, 2011 New Senior Credit Facility [Member] Letters Of Credit [Member]	Oct. 11, 2011 New Senior Credit Facility [Member] Revolving Credit Facility [Member]	Oct. 11, 2011 Amended Receivables Credit Facility Agreement [Member]	Dec. 31, 2011 Amended Receivables Credit Facility Agreement [Member]	Dec. 31, 2011 Receivables Credit Facility [Member]	Oct. 11, 2011 Receivables Credit Facility [Member]	Dec. 31, 2010 Receivables Credit Facility [Member]	Dec. 31, 2011 Receivables Credit Facility [Member] Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Offerings to qualified institutional investors				$ 150,000,000	$ 400,000,000			$ 150,000,000
Senior notes, stated percentage				4.375%	5.75%	5.75%	5.75%	6.50%	6.50%	6.50%
Senior notes						399,486,000	399,143,000		149,962,000	149,956,000
Debt instrument maturity						Aug. 01, 2013			Mar. 15, 2018
Debt Instrument Maturity Year				2008	2013			March 15, 2018
Credit facility, effective interest rate																		1.15%		1.56%
Credit facility, expiration date											April 2013	October 11, 2016				October 11, 2014		October 11, 2014
Credit facility borrowing capacity	450,000,000												150,000,000	50,000,000	250,000,000	200,000,000		200,000,000	150,000,000		200,000,000
Outstanding letters of credit	14,300,000											14,300,000
Unused borrowing capacity												235,700,000					91,000,000
Annual principal maturities for debt, 2012	15,000,000
Annual principal maturities for debt, year two	415,000,000
Annual principal maturities for debt, year three	124,000,000
Annual principal maturities for debt, year four	15,000,000
Annual principal maturities for debt, year five	90,000,000
Annual principal maturities for debt, after year five	150,000,000
Interest payments for debt obligations	35,200,000	35,300,000	35,700,000
Credit facility outstanding	123,312,000																	109,000,000
Outstanding principal balance, maximum																		$ 109,000,000