Debt Obligations Debt Obligations (Details Textuals) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Debt Instrument [Line Items]
Unsecured credit facility	$ 0		$ 69,000,000
Repayments of Unsecured Debt	0	37,500,000
Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate	6.65%	6.72%
Long-term Debt, Gross	441,177,000		443,942,000
Unsecured Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	1,958,953,000		2,027,953,000
Unsecured Debt [Member] | Three Year Term Loan [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	150,000,000
Derivative, Average Fixed Interest Rate			2.596%
Notional Amount of Interest Rate Cash Flow Hedge Derivatives			150,000,000
Long-term Debt, Gross	150,000,000		150,000,000
Unsecured Debt [Member] | Letter of Credit [Member]
Debt Instrument [Line Items]
Letters of Credit Outstanding, Amount	900,000
Unsecured Debt [Member] | Line of Credit [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Remaining Borrowing Capacity	599,100,000
Line of Credit Facility, Option for Additional Borrowing Capacity subject to customary conditions and limitations	200,000,000
Long-term Debt, Gross	0		69,000,000
Unsecured Debt [Member] | Four Year Term Loan [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	100,000,000
Unsecured Debt [Member] | Seven Year Term Loan [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	200,000,000
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		$ 200,000,000