Debt (Narrative) (Details) (USD $)	12 Months Ended		1 Months Ended	12 Months Ended
	Dec. 31, 2012	Feb. 07, 2012	Aug. 31, 2012 Unsecured Senior Notes Due 2014 [Member]	Dec. 31, 2012 Unsecured Senior Notes Due 2014 [Member]	Dec. 31, 2012 Unsecured Senior Notes Due 2019 [Member]	Dec. 31, 2012 Unsecured Senior Notes Due 2021 [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
Line of Credit Facility, Current Borrowing Capacity		600,000,000
Line of credit facility maximum borrowing capacity before amendment		400,000,000
Aggregate principal amount of term loan		400,000,000
Line of Credit Facility, Periodic Payment	5,000,000
Line of credit facility amendments cost	24,700,000
Letters of Credit Outstanding, Amount	49,000,000
Senior Notes				150,000,000	500,000,000	800,000,000
Stated interest rate on unsecured senior notes		8.00%	6.00%		6.375%	7.125%
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)			2,300,000
Extinguishment of Debt, Amount			$ 150,000,000