Fair Value Measurements (Details) (USD $)		12 Months Ended																								3 Months Ended			3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012 Fair Value, Measurements, Nonrecurring [Member]		Dec. 31, 2012 Minimum	Dec. 31, 2012 Minimum Overall cap rate [Member]	Dec. 31, 2012 Minimum Terminal cap rate [Member]	Dec. 31, 2012 Maximum	Dec. 31, 2012 Maximum Overall cap rate [Member]	Dec. 31, 2012 Maximum Terminal cap rate [Member]	Mar. 31, 2013 Estimate of Fair Value, Fair Value Disclosure [Member] Fair Value, Measurements, Recurring [Member]	Dec. 31, 2012 Estimate of Fair Value, Fair Value Disclosure [Member] Fair Value, Measurements, Recurring [Member]	Dec. 31, 2012 Estimate of Fair Value, Fair Value Disclosure [Member] Fair Value, Measurements, Nonrecurring [Member]	Mar. 31, 2013 Fair Value, Inputs, Level 1 [Member] Fair Value, Measurements, Recurring [Member]	Dec. 31, 2012 Fair Value, Inputs, Level 1 [Member] Fair Value, Measurements, Recurring [Member]	Dec. 31, 2012 Fair Value, Inputs, Level 1 [Member] Fair Value, Measurements, Nonrecurring [Member]	Mar. 31, 2013 Fair Value, Inputs, Level 2 [Member]	Dec. 31, 2012 Fair Value, Inputs, Level 2 [Member]	Mar. 31, 2013 Fair Value, Inputs, Level 2 [Member] Fair Value, Measurements, Recurring [Member]	Dec. 31, 2012 Fair Value, Inputs, Level 2 [Member] Fair Value, Measurements, Recurring [Member]	Dec. 31, 2012 Fair Value, Inputs, Level 2 [Member] Fair Value, Measurements, Nonrecurring [Member]	Mar. 31, 2013 Fair Value, Inputs, Level 3 [Member]	Dec. 31, 2012 Fair Value, Inputs, Level 3 [Member]	Mar. 31, 2013 Fair Value, Inputs, Level 3 [Member] Fair Value, Measurements, Recurring [Member]	Dec. 31, 2012 Fair Value, Inputs, Level 3 [Member] Fair Value, Measurements, Recurring [Member]	Dec. 31, 2012 Fair Value, Inputs, Level 3 [Member] Fair Value, Measurements, Nonrecurring [Member]	Mar. 31, 2013 Unsecured Credit Facilities [Member]	Mar. 31, 2013 Unsecured Credit Facilities [Member] Fair Value, Inputs, Level 2 [Member]	Dec. 31, 2012 Unsecured Credit Facilities [Member] Fair Value, Inputs, Level 2 [Member]	Mar. 31, 2013 Notes Payable, Other Payables [Member] Minimum	Mar. 31, 2013 Notes Payable, Other Payables [Member] Maximum	Mar. 31, 2013 Notes Receivable [Member] Minimum	Mar. 31, 2013 Notes Receivable [Member] Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-Lived Assets												$ 49,673,000			$ 0					$ 0					$ 49,673,000
Risk Free Interest Rate																										1.60%			2.30%	3.00%	7.01%	8.56%
Notes Receivable, Fair Value																					19,700,000	23,700,000
Notes Payable, Fair Value																2,020,000,000	2,000,000,000
Debt Instrument, Fair Value Disclosure																											145,200,000	170,200,000
Trading Securities, Fair Value Disclosure										24,495,000	23,429,000		24,495,000	23,429,000				0	0				0	0
Total										32,174,000	27,736,000		24,495,000	23,429,000				7,685,000	4,412,000				(6,000)	(105,000)
Impairment of Real Estate		54,500,000	[1]
Assets, Fair Value Disclosure, Nonrecurring	0
Interest Rate Derivative Assets, at Fair Value										7,679,000	4,307,000		0	0				7,685,000	4,412,000				(6,000)	(105,000)
Derivative instruments, at fair value										$ (64,000)	$ (76,000)		$ 0	$ 0				$ (65,000)	$ (77,000)				$ 1,000	$ 1,000
Fair Value Inputs, Cap Rate					8.30%	8.80%		8.50%	8.80%
Fair Value Inputs, Long-term Revenue Growth Rate				(8.30%)			2.50%
Fair Value Inputs, Discount Rate				10.50%			10.50%

[1] Excludes impairments for properties sold during the year ended December 31, 2012.