Debt (Details Textual)	1 Months Ended			9 Months Ended					1 Months Ended		1 Months Ended			1 Months Ended		9 Months Ended		1 Months Ended
	Jun. 30, 2013 USD ($)	Jun. 30, 2013 CAD	Jan. 31, 2013 USD ($)	Sep. 30, 2013 USD ($)	Sep. 30, 2012 USD ($)	Dec. 31, 2012	Sep. 30, 2013 Term Loan [Member] USD ($)	Oct. 31, 2013 Senior Notes [Member] Subsequent Event [Member] USD ($)	Jan. 31, 2013 Minimum [Member]	Sep. 30, 2013 Minimum [Member] Line of Credit [Member] USD ($)	Jan. 31, 2013 Minimum [Member] Term Loan [Member]	Jan. 31, 2013 Maximum [Member]	Sep. 30, 2013 Maximum [Member] Line of Credit [Member] USD ($)	Jan. 31, 2013 Maximum [Member] Term Loan [Member]	Jan. 31, 2013 Revolving Credit Facility [Member] USD ($)	Sep. 30, 2013 Revolving Credit Facility [Member] USD ($)	Dec. 31, 2012 Revolving Credit Facility [Member] USD ($)	Jan. 31, 2013 Revolving Credit Facility [Member] Minimum [Member]	Jan. 31, 2013 Revolving Credit Facility [Member] Maximum [Member]	Jan. 31, 2013 Standby Letters of Credit [Member] USD ($)
Loans Payable, Fair Value Disclosure																$ 277,100,000	$ 227,300,000
Leverage Ratio For Eurodollar Rate Based Borrowings											1.70%			2.45%				1.75%	2.50%
Leverage Ratio For Base Rate Based Borrowings											0.70%			1.45%				0.75%	1.50%
Debt, Weighted Average Interest Rate				3.35%		3.99%
Line of Credit Facility, Current Borrowing Capacity															15,000,000
Line of Credit Facility, Maximum Borrowing Capacity																27,500,000				30,000,000
Line of Credit Facility, Borrowing Capacity, Description																(a) the lesser of (i) 60.0% of the "as-is" appraised value of our properties that form the borrowing base of the Amended Credit Facility and (ii) the amount that would create a debt service coverage ratio of not less than 1.5, and (b) 50% of the aggregate of the lesser of (i) the book value of each of our development assets (as such term is defined in the Second Amended and Restated Credit Agreement) and (ii) the "as-is" appraised value of each of our development assets, subject to certain limitations in the Second Amended and Restated Credit Agreement.
Line of Credit Facility, Amount Outstanding							50,000,000									160,500,000
Long Term Line Of Credit			50,000,000
Future Commitment Line Of Credit Facility Maximum Borrowing Capacity										300,000,000			600,000,000
Line Of Credit Facility Covenant Terms, Leverage Ratio, Maximum									0.60			1.00
Line Of Credit Facility Covenant Term, Fixed Charge Coverage Ratio, Minimum									1.00			1.50
Line Of Credit Facility Covenant Terms, Fixed Rate Debt and Debt, Subject To Hedge Agreements To Total Debt Minimum, Percentage			66.67%
Line Of Credit Facility Covenant Terms, Secured Recourse Debt Ratio, Maximum, Percentage			20.00%
Line Of Credit Facility Covenant Terms, Tangible Minimum			330,788,250
Line Of Credit Facility Covenant Terms, Percentage Of Net Proceeds Of Additional Equity Issuances			75.00%
Line Of Credit Facility Covenant Terms, Percentage Secured Debt Ratio Description			not greater than 50% through February 17, 2013 and not greater than 45% on any date thereafter
Line Of Credit Facility Covenant Terms, Percentage Of Distributions Of Funds From Operations			95.00%
Line of Credit Facility, Increase, Additional Borrowings				189,376,000	43,900,000										250,000,000
Proceeds from Unsecured Notes Payable	33,400,000	35,000,000
Debt Instrument, Interest Rate During Period				1.22%
Debt Instrument, Maturity Date				Dec. 27, 2013
Line Of Credit Facility Private Offering								$ 100,000,000
Line Of Credit Facility Unsecured Exchange Rate								4.75%
Debt Instrument, Basis Spread on Variable Rate				1.70%