Debt Obligations (Details Textuals 2) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Debt Instrument [Line Items]
Long-term Line of Credit	$ 0	$ 69,000,000
Long Term Debt Maturities Repayments Of Principal	2,599,465,000
Extinguishment of Debt, Amount	29,250,000
Amortization of Financing Costs	4,676,000	6,208,000	4,991,000
Derivative, Gain (Loss) on Derivative, Net	0	(2,985,000)	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]
Line of Credit Facility, Interest Rate During Period	0.29%	1.02%
Long-term Debt, Gross	1,929,703,000	2,027,953,000
Unsecured Debt [Member] | Three Year Term Loan [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	1.75%
Derivative, Average Fixed Interest Rate	2.596%
Long-term Debt, Gross	150,000,000	150,000,000
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	100,000,000
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 three	1.00%
Derivative, Average Fixed Interest Rate	3.623%
Long-term Debt, Gross	200,000,000	200,000,000
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	250,000,000
Unsecured Debt [Member] | Note Five [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	157,625,000	166,535,000
Unsecured Debt [Member] | Note Six [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	149,919,000	150,429,000
Unsecured Debt [Member] | Letter of Credit [Member]
Debt Instrument [Line Items]
Letters of Credit Outstanding, Amount	1,300,000
Unsecured Debt [Member] | Line of Credit [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	1.50%
Long-term Debt, Gross	0	69,000,000
Line of Credit Facility, Remaining Borrowing Capacity	598,700,000
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	5.73%	6.65%	6.72%
Long-term Debt, Gross	669,762,000	443,942,000
Note Six [Member]
Debt Instrument [Line Items]
Extinguishment of Debt, Amount	$ 510,000