Subsequent Events Subsequent Events (Narrative) (Details) (USD $)	1 Months Ended	9 Months Ended				9 Months Ended				1 Months Ended			1 Months Ended
	Nov. 30, 2012 jvs	Sep. 30, 2013 sqft	Dec. 31, 2012	Feb. 29, 2012	Sep. 30, 2013 Cash Flow Hedges [Member] Forward-Starting Contracts [Member] contracts	Sep. 30, 2013 Unsecured Revolving Credit Facility [Member]	Sep. 30, 2013 Consoldiated Real Estate Joint Ventures [Member] jvs	Sep. 30, 2013 Unconsolidated Real Estate Joint Ventures [Member] jvs	Dec. 31, 2012 Unconsolidated Real Estate Joint Ventures [Member]	Nov. 06, 2013 Subsequent Event [Member] sqft	Nov. 06, 2013 Subsequent Event [Member] Cash Flow Hedges [Member] Forward-Starting Contracts [Member] contracts	Nov. 06, 2013 Subsequent Event [Member] Unsecured Revolving Credit Facility [Member]	Nov. 06, 2013 Subsequent Event [Member] 4.45% Unsecured Senior Notes [Member]	Nov. 06, 2013 Subsequent Event [Member] Consoldiated Real Estate Joint Ventures [Member] jvs	Nov. 06, 2013 Subsequent Event [Member] Unconsolidated Real Estate Joint Ventures [Member] jvs
Subsequent Event [Line Items]
Gross payments to acquire real estate		$ 73,100,000								$ 85,600,000
Square footage of operating properties		50,400,000								350,000
Notes receivable from real estate joint ventures and partnerships, currently due	16,100,000									16,100,000
Receipt of cash and real property										19,100,000
Noncontrolling interest, increase from settlement										(16,200,000)
Number of real estate joint ventures	3						1	2						2	2
Total assets		4,118,547,000	4,184,784,000					1,382,119,000	1,519,447,000	172,500,000					23,200,000
Ownership percentage in joint ventures			75.00%	47.80%				51.00%		30.00%
Note payable										100,000,000
Senior unsecured notes													250,000,000
Debt stated interest rate													4.45%
Debt maturity date						Apr. 18, 2017							Jan. 15, 2024
Debt issued discount rate													99.58%
Debt effective interest rate													4.50%
Proceeds from issuance of debt													247,300,000
Maximum borrowing capacity under credit facility						500,000,000						500,000,000
Number of active interest rate contracts held					3						3
Notional amount of interest rate cash flow hedge derivatives					150,000,000						150,000,000
Cash received on cash flow hedge											$ 6,100,000