Long-Term Debt Activity Under Revolving Credit Facility (Details) (USD $)	12 Months Ended			0 Months Ended	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Oct. 12, 2011 Revolving Credit Facility Member	Dec. 31, 2011 Revolving Credit Facility Member	Dec. 31, 2010 Revolving Credit Facility Member	Oct. 12, 2011 Libor Based Market Rate Spreads Member Revolving Credit Facility Member	Dec. 31, 2011 Libor Based Market Rate Spreads Member Minimum [Member] Revolving Credit Facility Member	Dec. 31, 2011 Libor Based Market Rate Spreads Member Maximum [Member] Revolving Credit Facility Member	Oct. 12, 2011 Lenders Base Rate Spreads Member Revolving Credit Facility Member	Dec. 31, 2011 Lenders Base Rate Spreads Member Minimum [Member] Revolving Credit Facility Member	Dec. 31, 2011 Lenders Base Rate Spreads Member Maximum [Member] Revolving Credit Facility Member
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity					$ 500,000,000
Letter Of Credit Sublimit					25,000,000
Swing Line Loan Sublimit					30,000,000
Additional Commitments With Accordion Feature					250,000,000
Payments of Debt Issuance Costs	3,873,000	4,997,000	0		3,873,000
Line of Credit Facility, Expiration Date				October 12, 2016
Debt Instrument, Basis Spread on Variable Rate							1.50%	1.25%	2.25%	0.50%	0.25%	1.25%
Debt, Weighted Average Interest Rate					0.00%	0.60%
Line of Credit Facility, Amount Outstanding					0	0
Line of Credit Facility, Average Outstanding Amount					0	38,200,000
Line of Credit Facility, Remaining Borrowing Capacity					498,800,000
Contingent Liability under Letters of Credit					$ 1,200,000	$ 1,300,000