Notes Payable and Unsecured Credit Facilities (Details) (USD $)								12 Months Ended									12 Months Ended								12 Months Ended		3 Months Ended	12 Months Ended					12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Oct. 07, 2010	Jun. 02, 2010	Dec. 31, 2012 Scheduled Principal Payments [Member]	Dec. 31, 2012 Mortgage Loan Maturities [Member]	Dec. 31, 2012 Unsecured Maturities [Member]	Dec. 31, 2012 Wholly Owned Properties [Member]	Dec. 31, 2011 Wholly Owned Properties [Member]	Dec. 31, 2012 Wholly Owned Properties [Member] Grand Ridge Plaza - Operating Component [Member]	Dec. 31, 2012 Wholly Owned Properties [Member] Sandy Springs [Member]	Dec. 31, 2012 Partnership Interest [Member]	Dec. 31, 2011 Partnership Interest [Member]	Dec. 31, 2010 Partnership Interest [Member]	Dec. 31, 2012 Notes Payable to Banks [Member]	Dec. 31, 2011 Notes Payable to Banks [Member]	Dec. 31, 2012 Fixed Rate Mortgage Loans [Member]	Dec. 31, 2011 Fixed Rate Mortgage Loans [Member]	Dec. 31, 2012 Variable Rate Mortgage Loans [Member]	Dec. 31, 2011 Variable Rate Mortgage Loans [Member]	Dec. 31, 2012 Fixed Rate Unsecured Loans [Member]	Dec. 31, 2011 Fixed Rate Unsecured Loans [Member]	Dec. 31, 2012 Unsecured Credit Facilities [Member]	Dec. 31, 2011 Unsecured Credit Facilities [Member]	Dec. 31, 2012 Secured Debt [Member]	Dec. 31, 2012 Secured Debt [Member] Partnership Interest [Member]	Mar. 31, 2011 Unsecured Debt [Member]	Dec. 31, 2012 Unsecured Debt [Member]	Dec. 31, 2012 Unsecured Debt [Member] Construction Loans [Member]	Dec. 31, 2012 Unsecured Debt [Member] General Loan [Member]	Dec. 31, 2012 Unsecured Debt [Member] Term Loan - Unsecured [Member]	Dec. 31, 2011 Unsecured Debt [Member] Term Loan - Unsecured [Member]	Dec. 31, 2012 Line of Credit [Member] Line of Credit Facility - Unsecured [Member]	Dec. 31, 2012 Variable Income Interest Rate [Member] Construction Loans [Member]
Debt Instrument [Line Items]
Secured Debt, Percent Guaranteed by Parent Company																										17.60%
Debt Instrument, Maturity Date Range, End																									Dec. 31, 2028			Dec. 31, 2021
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum																	5.22%											4.80%
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Maximum																	8.40%											6.00%
Debt, Weighted Average Interest Rate																	6.30%											5.46%
Mortgage Loans on Real Estate, Carrying Amount of Mortgages	$ 473,955,000																												$ 9,000,000	$ 3,000,000
Debt Instrument, Interest Rate Terms																														variable interest rate equal to LIBOR plus 380 basis points	variable interest rate of LIBOR plus 145 basis points			variable interest rate of LIBOR plus 160 basis points
Debt Instrument, Maturity Date																											Jan. 31, 2011		Sep. 01, 2014	Oct. 01, 2014	Dec. 15, 2016
Debt Instrument, Basis Spread on Variable Rate																														3.80%	1.45%		1.175%	1.60%
Line of Credit Facility, Commitment Fee Percentage																																	0.225%
Repayments of Long-term Debt																												192,400,000
Debt Instrument, Interest Rate, Stated Percentage			4.80%	6.00%																								7.00%
Noncash or Part Noncash Acquisition, Debt Assumed								30,467,000	31,292,000	12,810,000	17,657,000	30,467,000	31,292,000	58,981,000
Line of Credit Facility, Revolving, Maximum Borrowing Capacity																																	800,000,000
Line of Credit Facility, Increase, Additional Borrowings																																	200,000,000
Line of Credit Facility, Description																																	September 2016, subject to a one-year extension
Line of Credit Facility, Unsecured, Interest Rate Description																																	annual rate of LIBOR plus 117.5 basis points
Line of Credit Facility, Unsecured, Annual Facility Fee																																	22.5 basis points
Line of Credit Facility, Maximum Increase Allowed to Borrowing Capacity																																	1,000,000,000
Debt Instrument, Face Amount			250,000,000	150,000,000																											250,000,000
Debt Instrument, Decrease, Repayments																															150,000,000
Write off of Deferred Debt Issuance Cost																															852,000
Long-term Debt, Amount Outstanding	1,941,891,000	1,982,440,000			51,093,000	422,862,000	1,467,936,000								1,771,891,000	1,942,440,000	461,914,000	439,880,000	12,041,000	12,665,000	1,297,936,000	1,489,895,000	170,000,000	40,000,000							100,000,000	0
Debt Instrument, maximum increase allowed to borrowing capacity																															150,000,000
Maturities of Long-term Debt [Abstract]
2013	24,191,000				7,872,000	16,319,000	0
2014	184,382,000				7,383,000	26,999,000	150,000,000
2015	418,181,000				5,746,000	62,435,000	350,000,000
2016	189,648,000				5,487,000	14,161,000	170,000,000
2017	488,959,000				4,584,000	84,375,000	400,000,000
Beyond 5 Years	632,764,000				20,021,000	212,743,000	400,000,000
Unamortized debt discounts (premiums)	3,766,000				0	5,830,000	(2,064,000)
Long-term Debt	1,941,891,000	1,982,440,000			51,093,000	422,862,000	1,467,936,000								1,771,891,000	1,942,440,000	461,914,000	439,880,000	12,041,000	12,665,000	1,297,936,000	1,489,895,000	170,000,000	40,000,000							100,000,000	0
Line of Credit Facility, Amount Outstanding																																	$ 70,000,000