Debt Obligations (Details Textuals) (USD $)	9 Months Ended						9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Feb. 01, 2012 Unsecured Debt [Member]	Sep. 30, 2012 Letter of Credit [Member] Unsecured Debt [Member]	Sep. 30, 2012 New Revolving Credit Facility [Member] Unsecured Debt [Member]	Dec. 15, 2011 New Revolving Credit Facility [Member] Unsecured Debt [Member]	Sep. 30, 2012 Bank Term Loan and the Credit Facility [Member] Unsecured Debt [Member]	Sep. 30, 2012 $300.0M 5.750% Guaranteed Notes due 2012 [Member] Unsecured Debt [Member]	Sep. 30, 2012 $250.0M 5.400% Guaranteed Notes due 2014 [Member] Unsecured Debt [Member]	Sep. 30, 2012 $250.0M 7.500% Guaranteed Notes due 2015 [Member] Unsecured Debt [Member]	Sep. 30, 2012 $250.0M 6.000% Guaranteed Notes due 2016 [Member] Unsecured Debt [Member]	Sep. 30, 2012 $300.0M 5.700% Guaranteed Notes due 2017 [Member] Unsecured Debt [Member]	Sep. 30, 2012 $325.0M 4.950% Guaranteed Notes due 2018 [Member] Unsecured Debt [Member]	Feb. 01, 2012 Three Year Term Loan [Member] Unsecured Debt [Member]	Feb. 01, 2012 Four Year Term Loan [Member] Unsecured Debt [Member]	Feb. 01, 2012 Seven Year Term Loan [Member] Unsecured Debt [Member]
Debt obligations (Textuals)
Line of Credit Facility, Maximum Borrowing Capacity			$ 600,000,000
Debt instrument, face amount			600,000,000					300,000,000	250,000,000	250,000,000	250,000,000	300,000,000	325,000,000	150,000,000	250,000,000	200,000,000
Repayments of Unsecured Debt	37,500,000	0					183,000,000
Letters of credit outstanding				2,800,000
Option for additional borrowings subject to customary conditions and limitations						200,000,000
Unused availability under the Credit Facility					$ 597,200,000