Notes and Bonds Payable (Details) (USD $)	3 Months Ended		9 Months Ended		0 Months Ended				9 Months Ended	0 Months Ended	1 Months Ended	0 Months Ended	3 Months Ended	0 Months Ended		9 Months Ended	1 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Jun. 28, 2013 TIAA 7.25% secured loan	Apr. 18, 2013 Senior Notes [Member] 5.125% Senior Notes Due 2014 [Member]	Mar. 26, 2013 Senior Notes [Member] 3.75% Senior Notes Due 2023 [Member]	Mar. 26, 2013 Senior Notes [Member] 3.75% Senior Notes Due 2023 [Member] Other Assets	Sep. 30, 2013 Senior Notes [Member] Senior Notes Due 2017 [Member]	Jun. 03, 2013 Mortgages [Member]	Feb. 28, 2013 Mortgages [Member] 6.55% Mortage Note Payable [Member]	Feb. 15, 2013 Line of Credit [Member] Unsecured Credit Facility Due 2015 [Member] extension_option	Sep. 30, 2013 Line of Credit [Member] Unsecured Credit Facility Due 2015 [Member]	Feb. 15, 2013 Line of Credit [Member] Unsecured Credit Facility Due 2015 [Member] Minimum	Feb. 15, 2013 Line of Credit [Member] Unsecured Credit Facility Due 2015 [Member] Maximum	Sep. 30, 2013 Real Estate Acquisitions	Sep. 30, 2013 Real Estate Acquisitions COLORADO		Oct. 31, 2013 Real Estate Acquisitions WASHINGTON Subsequent Event	Oct. 31, 2013 Real Estate Acquisitions NORTH CAROLINA Subsequent Event
Debt Instrument [Line Items]
Mortgage Notes Payable Assumed																$ 12,000,000	$ 12,000,000	[1]	$ 16,600,000	$ 11,200,000
Fair value adjustment premium on liabilities assumed																	700,000
Number of extension options												2
Period of extension option												6 months
Extension fees as percentage of aggregate commitments												0.075%
Percentage of interest in addition to LIBOR												1.40%		0.95%	1.75%
Facility fee on the aggregate amount of commitments												0.30%		0.15%	0.35%
Deferred finance costs						200,000						2,700,000
Write off of unamortized deferred financing costs					500,000								300,000
Amount outstanding under credit facility													185,000,000
Remaining borrowing capacity													515,000,000
Repayments of Notes Payable										1,100,000	14,900,000
Debt face amount							250,000,000.0
Debt interest rate					7.25%	5.125%	3.75%				6.55%						6.17%		6.01%	5.86%
Debt Instrument, Unamortized Discount							2,100,000
Debt yielded interest rate							3.85%										5.25%
Unamortized Debt Issuance Expense								2,100,000
Extinguishment of debt amount					94,300,000	277,300,000
Extinguishment of debt outstanding principal amount					77,000,000	264,700,000
Extinguishment of debt, accrued interest amount					500,000	700,000
Prepayment penalties					16,800,000					300,000
Make whole amount on extinguishment of debt						11,900,000
Write off of unamortized discount						200,000
Loss on extinguishment of debt	0	0	(29,638,000)	0	17,400,000	12,300,000
Authorized Repurchase Of Debt Amount									$ 50,000,000

[1] The mortgage note payable assumed in the acquisition does not reflect the fair value adjustment of $0.7 million recorded by the Company upon acquisition (included in Other).