Schedule I Disclosures for Parent Company Only Schedule I, Notes Payable (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Short-term Debt [Line Items]
Notes payable	$ 345,000	$ 249,000
Letters of Credit Outstanding, Amount	43,700	46,865
Parent [Member]
Short-term Debt [Line Items]
Notes payable	345,000	249,000
Parent [Member] | Line of Credit [Member] | Revolving Credit Facility, Existing [Member]
Short-term Debt [Line Items]
Line of Credit Facility, Expiration Date	4/14/2013
Notes payable	195,000	149,000
Letters of Credit Outstanding, Amount	43,700	46,865
Line of Credit Facility, Commitment Fee Percentage	0.50%
Parent [Member] | Loans Payable [Member] | Corporate Term Loan due June 2012 [Member]
Short-term Debt [Line Items]
Notes payable	150,000	0
Letters of Credit Outstanding, Amount	$ 0	$ 0
Term Loan, Maturity Date	Jun. 24, 2012
Debt Instrument, Basis Spread on Variable Rate	1.25%
Debt Instrument, Interest Rate at Period End	1.56%
Parent [Member] | Base Rate Option [Member] | Line of Credit [Member] | Revolving Credit Facility, Existing [Member]
Short-term Debt [Line Items]
Line of Credit Facility, Interest Rate at Period End	1.75%
Parent [Member] | Eurodollar Option [Member] | Line of Credit [Member] | Revolving Credit Facility, Existing [Member]
Short-term Debt [Line Items]
Line of Credit Facility, Interest Rate at Period End	2.75%
Parent [Member] | Letters of Credit [Member] | Line of Credit [Member] | Revolving Credit Facility, Existing [Member]
Short-term Debt [Line Items]
Line of Credit Facility, Interest Rate at Period End	2.75%