Debt (Details) (USD $)	3 Months Ended	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2012
Debt Instrument [Line Items]
Line of Credit Facility, Current Borrowing Capacity	$ 600,000,000	$ 600,000,000
Line of credit facility maximum borrowing capacity before amendment	400,000,000	400,000,000
Aggregate principal amount of term loan	400,000,000	400,000,000
Line of Credit Facility, Periodic Payment		5,000,000
Line of credit facility amendments cost	24,700,000	24,700,000
Letters of Credit Outstanding, Amount	49,300,000	49,300,000
Stated interest rate on unsecured senior notes	8.00%	8.00%
Repayment Of Us Government Guaranteed Long Term Financing Connected To Disposition		12,500,000
Make Whole Premiums Connected To Dispositions		4,000,000
Written Off Unamortized Loan Costs Connected To Repayments Of Make Whole Premiums		700,000
Gains (Losses) on Extinguishment of Debt	(2,294,000)	(2,294,000)
Line of Credit Facility, Amount Outstanding	90,000,000	90,000,000
Unsecured Senior Notes Due 2014 [Member]
Debt Instrument [Line Items]
Face value of debt extinguished	100
Portion of debt extinguished	50
Original face amount of debt instrument	300,000,000	300,000,000
Senior Notes	150,000,000	150,000,000
Stated interest rate on unsecured senior notes	6.00%	6.00%
Gains (Losses) on Extinguishment of Debt	2,300,000
Extinguishment of Debt, Amount	150,000,000
Unsecured Senior Notes Due 2019 [Member]
Debt Instrument [Line Items]
Senior Notes	500,000,000	500,000,000
Stated interest rate on unsecured senior notes	6.375%	6.375%
Unsecured Senior Notes Due 2021 [Member]
Debt Instrument [Line Items]
Senior Notes	$ 800,000,000	$ 800,000,000
Stated interest rate on unsecured senior notes	7.125%	7.125%