Notes and Bonds Payable (Details Textual) (USD $)	6 Months Ended				6 Months Ended					6 Months Ended			6 Months Ended			6 Months Ended					6 Months Ended								6 Months Ended
	Jun. 30, 2012 MortgageLoan	Jun. 30, 2012 Unsecured Credit Facility [Member]	Dec. 31, 2011 Unsecured Credit Facility [Member]	Oct. 14, 2011 Unsecured Credit Facility [Member] Lender	Jun. 30, 2012 Unsecured Credit Facility [Member] Maximum [Member]	Jun. 30, 2012 Unsecured Credit Facility [Member] Minimum [Member]	Jun. 30, 2012 Senior Notes Due 2014 [Member]	Dec. 31, 2011 Senior Notes Due 2014 [Member]	Dec. 31, 2004 Senior Notes Due 2014 [Member]	Jun. 30, 2012 Senior Notes Due 2017 [Member]	Dec. 31, 2011 Senior Notes Due 2017 [Member]	Dec. 04, 2009 Senior Notes Due 2017 [Member]	Jun. 30, 2012 Senior Notes Due 2021 [Member]	Dec. 31, 2011 Senior Notes Due 2021 [Member]	Dec. 13, 2010 Senior Notes Due 2021 [Member]	Jun. 30, 2012 Contractual interest rates [Member]	Jun. 30, 2012 Mortgage Notes 6.480% [Member] Commercial Bank [Member]		Jun. 30, 2012 Mortgage Notes 6.110% [Member] Commercial Bank [Member]		Jun. 30, 2012 Mortgage Notes 7.650% [Member] Commercial Bank [Member]		Jun. 30, 2012 Mortgage Notes 6.810% [Member] Life Insurance Co. [Member]		Jun. 30, 2012 Mortgage Notes 6.430% [Member] Commercial Bank [Member]		Jun. 30, 2012 Mortgage Notes 7.250% [Member] Investment Fund [Member]		Jun. 30, 2012 Mortgage Notes 6.550% [Member] Investment Co. [Member]		Jun. 30, 2012 Mortgage Notes 4.700% [Member] Life Insurance Co. [Member]	Jun. 30, 2012 Mortgage Notes 4.700% [Member] Life Insurance Company - I [Member]	Jun. 30, 2012 Mortgage Notes 5.100% [Member] Insurance Co. [Member]		Jun. 30, 2012 Mortgage Notes 4.540% [Member] Commercial Bank [Member]		Jun. 30, 2012 Mortgage Notes 4.060% [Member] Life Insurance Co. [Member] MortgageLoan		Jun. 30, 2012 Mortgage Notes 4.060% [Member] Life Insurance Company - I [Member]		Jun. 30, 2012 Mortgage Notes Payable [Member] MortgageLoan	Dec. 31, 2011 Mortgage Notes Payable [Member]
Notes and Bonds Payable (Textual) [Abstract]
Maturity Date		Oct. 14, 2015					Apr. 01, 2014			Jan. 17, 2017			Jan. 15, 2021
Amount outstanding under Unsecured Credit Facility		$ 216,000,000	$ 212,000,000
Unsecured credit facility				700,000,000
Percentage of interest in addition to LIBOR		1.50%			1.90%	1.075%
Extension fees as percentage of aggregate commitments		0.20%
Facility fee on the aggregate amount of commitments		0.35%			0.45%	0.175%
Weighted average interest rate (LOC)		1.75%
Remaining borrowing capacity		484,000,000
Number of Lenders of unsecured credit facility				17
Extension year for unsecured credit facility due 2015	one
Public Issue of Unsecured senior notes									300,000,000			300,000,000			400,000,000
Interest rate on senior notes							5.125%			6.50%			5.75%																6.75%
Original Discount on Note							1,500,000			2,000,000			3,200,000
Unaccreted discount							(292,000)	(366,000)		(1,406,000)	(1,535,000)		(2,822,000)	(2,948,000)			(2,700,000)		(2,100,000)		(2,400,000)		(200,000)		(1,000,000)																(3,558,000)	(3,728,000)
Interest rate yielded per annum upon issuance							5.19%			6.618%			5.855%				6.48%	[1],[2]	6.11%	[1],[3]	7.65%	[1],[4]	6.81%	[1],[5]	6.43%	[1],[6]	7.25%	[1]	6.55%	[1],[7]			5.10%	[1],[8]	4.54%	[1],[9]	4.06%	[1],[10],[11]	4.06%	[1],[9]
Repurchase of debt							35,300,000
Premium on notes issued																													$ 500,000		$ 300,000	$ 400,000	$ 600,000		$ 200,000		$ 300,000		$ 200,000
Number of Mortgage Notes Receivable Outstanding																																					2
Contractual interest rates, minimum																5.00%
Contractual interest rates, maximum																7.625%
Debt instrument principal and interest amortization period																					11 years
Period for monthly installments of interest																					24 months
Period for extension of initial term																													3
Number of outstanding mortgage notes	19																																								13
Option to extend maturity	two, one-year floating rate extension terms

[1] The contractual interest rates for the 19 outstanding mortgage notes ranged from 5.00% to 7.625% at June 30, 2012.

[2] The unaccreted portion of a $2.7 million discount recorded on this note upon acquisition is included in the balance above.

[3] The unaccreted portion of a $2.1 million discount recorded on this note upon acquisition is included in the balance above.

[4] The unaccreted portion of a $2.4 million discount recorded on this note upon acquisition is included in the balance above.

[5] The unaccreted portion of a $0.2 million discount recorded on this note upon acquisition is included in the balance above.

[6] The unaccreted portion of a $1.0 million discount recorded on this note upon acquisition is included in the balance above.

[7] The unamortized portion of a $0.5 million premium recorded on this note upon acquisition is included in the balance above.

[8] The unamortized portion of a $0.6 million premium recorded on this note upon acquisition is included in the balance above.

[9] The unamortized portion of a $0.2 million premium recorded on this note upon acquisition is included in the balance above.

[10] The unamortized portions of a $0.3 million premium recorded on these notes upon acquisition are included in the balance above.

[11] The balance consists of two notes secured by the same building.