Debt (Details) (USD $)	12 Months Ended			0 Months Ended		12 Months Ended							12 Months Ended					0 Months Ended					12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Oct. 19, 2012 Medium-term Notes [Member]	Jun. 03, 2011 Medium-term Notes [Member]	Dec. 31, 2013 Issuance of Debt, January [Member]	Dec. 31, 2013 Issuance of Debt, Maturity Tranche One, January [Member]	Dec. 31, 2013 Issuance of Debt, Maturity Tranche Two, January [Member]	Dec. 31, 2013 Issuance of Debt, October [Member]	Dec. 31, 2013 Issuance of Debt, Maturity Tranche One, October [Member]	Dec. 31, 2013 Issuance of Debt, Maturity Tranche Two, October [Member]	Dec. 31, 2012 Variable Rate Debt Hedged [Member]	Dec. 31, 2013 Project Specific Loans [Member] Mortgages [Member] Loans	Dec. 31, 2012 Project Specific Loans [Member] Mortgages [Member] Loans	Dec. 31, 2013 Construction Loan - Maryland [Member] Mortgages [Member] Loans	Dec. 31, 2013 Construction Loan - Texas [Member] Mortgages [Member] Loans	Dec. 31, 2012 Cross Collateralized and Cross Defaulted Loans [Member] Mortgages [Member] Loans	Jun. 19, 2013 Revolving Credit Facility [Member]	Jan. 12, 2012 Revolving Credit Facility [Member]	Dec. 31, 2013 Revolving Credit Facility [Member]	Dec. 31, 2012 Revolving Credit Facility [Member]	Jan. 12, 2012 Line of Credit [Member]	Dec. 31, 2013 LIBOR Rate [Member] Revolving Credit Facility [Member]	Dec. 31, 2013 Prime Based Rate [Member] Revolving Credit Facility [Member]	Dec. 31, 2013 Loans Repaid [Member]		Dec. 31, 2012 Loans Repaid [Member]
Debt Instrument [Line Items]
Real estate pledged as collateral	$ 389,100,000	$ 418,400,000
Cash paid for interest net of capitalized interest	24,100,000	29,100,000	30,800,000
Interest costs capitalized	3,600,000	1,500,000	740,000
Number of mortgage notes payable													13	15	1	1	5
Number Of Mortgage Notes Repaid														10
Loans Repaid																									129,319,000		178,200,000
Interest Rate																									6.10%	[1]	5.80%	[1]
Senior unsecured revolving credit facility, maximum borrowing capacity																			350,000,000.0
Line of credit, borrowing capacity																						250,000,000.0
Line of credit facility, contractual term																			4 years
Line of credit facility, contractual term extension																			1 year
Debt procurement cost				600,000		1,000,000			950,000									1,200,000	2,300,000
Description of variable rate basis																							LIBOR-based rate	Prime-based rate
Interest rate																							1.30%
Basis spread on variable rate																								0.30%
Outstanding borrowings, line of credit facility																				133,500,000	190,500,000
Weighted average interest rate	3.80%	3.70%				4.27%			4.65%			1.90%								1.47%	1.71%
Unsecured debt, face amount					$ 125,000,000.0	$ 150,000,000	$ 63,000,000.0	$ 87,000,000.0	$ 100,000,000	$ 45,000,000.0	$ 55,000,000.0
Debt Instrument, Term							8 years	10 years		7 years	10 years 2 months
Debt Instrument, Interest Rate, Stated Percentage							4.02%	4.45%		4.29%	4.94%
Debt, Weighted Average Term						9 years 2 months			8 years 9 months

[1] Represents weighted average interest rate for the loans listed.