Debt Obligations (Details Textuals 2) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Debt Instrument [Line Items]
Long-term Line of Credit	$ 69,000,000	$ 275,500,000
Long Term Debt Maturities Repayments Of Principal	2,471,895,000
Extinguishment of Debt, Amount	164,968,000
Amortization of Financing Costs	6,208,000	4,991,000	3,770,000
Derivative Instruments, Gain (Loss) Recognized in Income, Net	(2,985,000)	0	0
Bank Term Loan And The Credit Facility [Member] | Federal Funds Rate [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	0.50%
Unsecured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate		0.99%
Line of Credit Facility, Interest Rate During Period	2.83%	1.01%
Long-term Debt, Gross	2,027,953,000	1,888,111,000
Unsecured Debt [Member] | Three Year Term Loan [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	1.75%
Notional amount of interest rate cash flow hedge derivatives	150,000,000
Derivative, Average Fixed Interest Rate	2.596%
Long-term Debt, Gross	150,000,000	0
Unsecured Debt [Member] | Four-Year Term Loan - Variable [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	1.75%
Long-term Debt, Gross	100,000,000	0
Unsecured Debt [Member] | Four-Year Term Loan - Swapped to fixed [Member]
Debt Instrument [Line Items]
Long Term Debt Maturities Repayments Of Principal	150,000,000
Derivative Instruments, Gain (Loss) Recognized in Income, Net	3,000,000
Unsecured Debt [Member] | Seven Year Term Loan [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	1.90%
Declining prepayment penalty in year one	3.00%
Declining prepayment penalty in year two	2.00%
Declining prepayment penalty in year three	1.00%
Derivative, Average Fixed Interest Rate	3.623%
Long-term Debt, Gross	200,000,000	0
Unsecured Debt [Member] | Three and Four Year Term Loan [Member]
Debt Instrument [Line Items]
Derivative, lower remaining maturity range	3 years
Derivative, higher remaining maturity range	5 years
Unsecured Debt [Member] | Note 2023 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	250,000,000	0
Unsecured Debt [Member] | Note Five [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	166,535,000	227,329,000
Unsecured Debt [Member] | Note Six [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	150,429,000	250,000,000
Unsecured Debt [Member] | Letter of Credit [Member]
Debt Instrument [Line Items]
Long-term Line of Credit	69,000,000
Letters of Credit Outstanding, Amount	900,000
Unsecured Debt [Member] | Line of Credit [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	1.50%
Long-term Debt, Weighted Average Interest Rate	1.02%
Long-term Debt, Gross	69,000,000	0
Line of Credit Facility, Remaining Borrowing Capacity	530,100,000
Line of Credit Facility, Option for Additional Borrowing Capacity subject to customary conditions and limitations	200,000,000
Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate	6.65%	6.72%	6.59%
Long-term Debt, Gross	443,942,000	512,391,000
Senior Notes Due2015 [Member]
Debt Instrument [Line Items]
Extinguishment of Debt, Amount	$ 60,794,000