Mortgages Notes Payable, Revolving Credit Facility, Interest and Amortization of Deferred Debt Costs - Additional Information (Detail) (USD $)			1 Months Ended					1 Months Ended			6 Months Ended		6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Mar. 19, 2013 Hampshire Langley	May 28, 2013 Beacon Center	Feb. 27, 2013 Metro Pike Center	Feb. 27, 2013 Northrock	Jun. 30, 2013 Northrock Loan	Apr. 10, 2013 Cruse Market Place	Jun. 30, 2013 Fixed Rate Mortgage Notes Payable Metro Pike Center	Jun. 30, 2013 Fixed Rate Mortgage Notes Payable Clarendon Center Mortgages	Jun. 30, 2013 Unsecured Revolving Credit Facility	Dec. 31, 2012 Unsecured Revolving Credit Facility	Jun. 30, 2013 Unsecured Revolving Credit Facility Minimum	Jun. 30, 2013 Unsecured Revolving Credit Facility Maximum	Jun. 30, 2013 Letter of Credit
Debt Instrument [Line Items]
Debt outstanding	$ 826,200,000	$ 827,800,000										$ 38,000,000
Debt outstanding with fixed-rate	795,800,000	774,800,000
Debt outstanding with variable-rate	30,400,000	53,000,000
Carrying value of properties collateralizing mortgage notes	911,100,000	916,100,000
Line of credit facility, maximum borrowing capacity											175,000,000				404,000
Line of credit facility maturity date											May 20, 2016
Extension in line of credit facility period											1 year
Line of credit facility, maximum borrowing capacity											166,700,000
Interest rate spread on LIBOR					1.65%	1.65%					1.90%		1.60%	2.50%
Guarantor obligations, maximum exposure, percentage							50.00%
Debt outstanding							14,900,000		7,800,000
Guarantor obligations, maximum exposure							7,500,000		15,500,000	27,600,000
Debt instrument amount			18,000,000	35,000,000	15,600,000	15,000,000
Debt instrument maturity term			15 years	15 years	3 years	3 years
Debt instrument maturity date			2028	2028	2016	2016
Principal amortization period			25 years	20 years	25 years	25 years
Debt instrument required final principal payment			9,500,000	11,300,000	14,700,000	14,200,000
Repayment of debt					15,900,000	15,000,000		6,900,000
Loan extension period					2 years	2 years
Debt instrument maturity date					2013-06	2013-05
Debt instrument, fixed interest rate			4.04%	3.51%
Debt instrument, monthly principal and interest payment			$ 95,400	$ 203,200