Notes Payable and Unsecured Credit Facilities (Details) - USD ($)		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Oct. 01, 2015	Sep. 01, 2015	Aug. 01, 2015	Dec. 31, 2014	Mar. 07, 2014	Feb. 14, 2014
Debt Instrument [Line Items]
Document Period End Date		Sep. 30, 2015
Line of Credit Facility, Expiration Date		May 13, 2019
Total debt outstanding		$ 2,023,652,000					$ 2,021,357,000
Document Fiscal Year Focus		2015
Maturities of Long-term Debt [Abstract]
2017		$ (2,000)
Debt Assumed, Net of Premiums			$ 78,049,000
Fixed Rate Mortgage Loans [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate		5.20%
Total notes payable		$ 479,413,000					518,993,000
Maturities of Long-term Debt [Abstract]
Total notes payable		479,413,000					518,993,000
Notes Payable to Banks [Member]
Debt Instrument [Line Items]
Total notes payable		1,808,652,000					1,946,357,000
Maturities of Long-term Debt [Abstract]
Total notes payable		$ 1,808,652,000					1,946,357,000
Variable Rate Mortgage Loans [Member]
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate		1.10%
Total notes payable		$ 33,163,000					29,839,000
Maturities of Long-term Debt [Abstract]
Total notes payable		33,163,000					29,839,000
Fixed Rate Unsecured Loans [Member]
Debt Instrument [Line Items]
Total notes payable		1,296,076,000					1,397,525,000
Maturities of Long-term Debt [Abstract]
Total notes payable		1,296,076,000					1,397,525,000
Unsecured Debt [Member]
Debt Instrument [Line Items]
Total credit facilities		215,000,000					75,000,000
Term Loan [Member]
Debt Instrument [Line Items]
Term Loan, Withdrawals		$ 90,000,000
Line of Credit [Member]
Debt Instrument [Line Items]
Debt Instrument, Description of Variable Rate Basis		LIBOR
Line of Credit Facility, Commitment Fee Percentage		9250.00%
Debt instrument, Facility Fee basis points		$ 15
Proceeds from Lines of Credit		50,000,000
Borrowing capacity		800,000,000
Line of Credit [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Total credit facilities		50,000,000					0
Term Loan [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Total credit facilities		165,000,000					$ 75,000,000
Parent Company [Member]
Debt Instrument [Line Items]
Debt repaid		350,000,000	$ 150,000,000
5.650% & 5.360% maturing in 2015 [Member]
Debt Instrument [Line Items]
Repayments of Debt		$ 76,000,000
3.90% unsecured maturing 11/1/2025 [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Debt Instrument, Maturity Date		Nov. 01, 2025
Debt Instrument, Face Amount						$ 250,000,000
Debt Instrument, Interest Rate, Stated Percentage						3.90%
5.25% unsecured maturing 8/1/15 [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage						5.25%
Maturities of Long-term Debt [Abstract]
2015		$ 350,000,000
University Commons [Member]
Maturities of Long-term Debt [Abstract]
Debt Assumed, Net of Premiums					$ 42,799,000
Fairfield Portfolio [Member]
Maturities of Long-term Debt [Abstract]
Debt Assumed, Net of Premiums	[1]							$ 77,730,000
Shops at Mira Vista [Member]
Maturities of Long-term Debt [Abstract]
Debt Assumed, Net of Premiums									$ 319,000
Scheduled Principal Payments [Member]
Debt Instrument [Line Items]
Total notes payable		40,006,000
Maturities of Long-term Debt [Abstract]
Long-term Debt, Maturities, Repayments of Principal, Remainder of Fiscal Year		2,127,000
2016		6,603,000
2017		5,937,000
2018		5,021,000
2019		4,044,000
Beyond 5 Years		16,274,000
Unamortized debt (discounts) premiums, net		0
Total notes payable		40,006,000
Mortgage Loan Maturities [Member]
Debt Instrument [Line Items]
Total notes payable		472,570,000
Maturities of Long-term Debt [Abstract]
Long-term Debt, Maturities, Repayments of Principal, Remainder of Fiscal Year		0
2016		41,374,000
2017		116,383,000
2018		57,253,000
2019		106,000,000
Beyond 5 Years		141,362,000
Unamortized debt (discounts) premiums, net		10,198,000
Total notes payable		472,570,000
Unsecured Maturities [Member]
Debt Instrument [Line Items]
Total notes payable	[2]	1,511,076,000
Maturities of Long-term Debt [Abstract]
Long-term Debt, Maturities, Repayments of Principal, Remainder of Fiscal Year	[2]	0
2016	[2]	0
2017	[2]	400,000,000
2018	[2]	0
2019	[2]	215,000,000
Beyond 5 Years	[2]	900,000,000
Unamortized debt (discounts) premiums, net	[2]	(3,924,000)
Total notes payable	[2]	1,511,076,000
Long-term Debt [Member]
Debt Instrument [Line Items]
Total notes payable		2,023,652,000
Maturities of Long-term Debt [Abstract]
Long-term Debt, Maturities, Repayments of Principal, Remainder of Fiscal Year		2,127,000
2016		47,977,000
2017		522,320,000
2018		62,274,000
2019		325,044,000
Beyond 5 Years		1,057,636,000
Unamortized debt (discounts) premiums, net		6,274,000
Total notes payable		$ 2,023,652,000
Subsequent Event [Member] | 5.875% unsecured maturing 6/15/17 [Member] [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount				$ 100,000,000

[1] On March 7, 2014, the Company acquired an 80% controlling interest in the Fairfield Portfolio, consisting of three operating properties. As a result of consolidation, the Company recorded the non-controlling interest of $15.4 million at fair value. The portfolio consists of three operating properties located in Fairfield, CT.

[2] Includes unsecured public debt and unsecured credit facilities.