Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified			9 Months Ended		12 Months Ended		9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended																								9 Months Ended	12 Months Ended				9 Months Ended				12 Months Ended				3 Months Ended		9 Months Ended					12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013 Fair Value, Measurements, Nonrecurring [Member]		Dec. 31, 2012 Fair Value, Measurements, Nonrecurring [Member]		Sep. 30, 2013 Minimum	Dec. 31, 2012 Minimum	Sep. 30, 2013 Minimum Overall cap rate [Member]	Dec. 31, 2012 Minimum Overall cap rate [Member]	Sep. 30, 2013 Minimum Terminal 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]	Sep. 30, 2013 Estimate of Fair Value Measurement [Member] Fair Value, Measurements, Recurring [Member]	Dec. 31, 2012 Estimate of Fair Value Measurement [Member] Fair Value, Measurements, Recurring [Member]	Sep. 30, 2013 Estimate of Fair Value Measurement [Member] Fair Value, Measurements, Nonrecurring [Member]	Dec. 31, 2012 Estimate of Fair Value Measurement [Member] Fair Value, Measurements, Nonrecurring [Member]	Sep. 30, 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]	Sep. 30, 2013 Fair Value, Inputs, Level 1 [Member] Fair Value, Measurements, Nonrecurring [Member]	Dec. 31, 2012 Fair Value, Inputs, Level 1 [Member] Fair Value, Measurements, Nonrecurring [Member]	Sep. 30, 2013 Fair Value, Inputs, Level 2 [Member]	Dec. 31, 2012 Fair Value, Inputs, Level 2 [Member]	Sep. 30, 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]	Sep. 30, 2013 Fair Value, Inputs, Level 2 [Member] Fair Value, Measurements, Nonrecurring [Member]	Dec. 31, 2012 Fair Value, Inputs, Level 2 [Member] Fair Value, Measurements, Nonrecurring [Member]	Sep. 30, 2013 Fair Value, Inputs, Level 3 [Member]	Dec. 31, 2012 Fair Value, Inputs, Level 3 [Member]	Sep. 30, 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]	Sep. 30, 2013 Fair Value, Inputs, Level 3 [Member] Fair Value, Measurements, Nonrecurring [Member]	Dec. 31, 2012 Fair Value, Inputs, Level 3 [Member] Fair Value, Measurements, Nonrecurring [Member]	Sep. 30, 2013 Unsecured Credit Facilities [Member] Minimum	Dec. 31, 2012 Unsecured Credit Facilities [Member] Minimum	Dec. 31, 2012 Unsecured Credit Facilities [Member] Maximum	Sep. 30, 2013 Unsecured Credit Facilities [Member] Fair Value, Inputs, Level 2 [Member]	Dec. 31, 2012 Unsecured Credit Facilities [Member] Fair Value, Inputs, Level 2 [Member]	Sep. 30, 2013 Notes Payable, Other Payables [Member] Minimum	Sep. 30, 2013 Notes Payable, Other Payables [Member] Maximum	Sep. 30, 2013 Notes Receivable [Member] Minimum	Sep. 30, 2013 Notes Receivable [Member] Maximum	Dec. 31, 2012 Vine at Castaic [Member] Fair Value, Measurements, Nonrecurring [Member]		Dec. 31, 2012 Deer Springs [Member] Fair Value, Measurements, Nonrecurring [Member]		Sep. 30, 2013 Partnership Interest [Member]	Sep. 30, 2012 Partnership Interest [Member]	Sep. 30, 2013 Partnership Interest [Member]	Sep. 30, 2012 Partnership Interest [Member]	Dec. 31, 2012 Partnership Interest [Member]	Sep. 30, 2013 .76% Derivative [Member] Estimate of Fair Value Measurement [Member] Fair Value, Measurements, Recurring [Member]	Dec. 31, 2012 .76% Derivative [Member] Estimate of Fair Value Measurement [Member] Fair Value, Measurements, Recurring [Member]	Dec. 31, 2012 Notes Payable to Banks [Member] Minimum	Dec. 31, 2012 Notes Payable to Banks [Member] Maximum	Dec. 31, 2012 Notes Receivable [Member] Minimum	Dec. 31, 2012 Notes Receivable [Member] Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes receivable	$ 26,128	$ 23,751																																															$ 26,128		$ 26,128		$ 23,751
Long-Lived Assets																		4,686	49,673			0	0					0	0					4,686	49,673
Risk Free Interest Rate																																				1.50%	1.60%	1.60%			2.40%	3.60%	5.00%	7.70%												2.40%	3.30%	7.00%	8.10%
Notes Receivable, Fair Value																														25,755	23,677
Trading securities held in trust, at fair value																25,421	23,429																																25,421		25,421		23,429
Notes Payable, Fair Value																								1,930,004	2,000,000
Unsecured credit facilities	100,000	170,000																																															100,000		100,000		170,000
Debt Instrument, Fair Value Disclosure																																							100,476	170,200
Trading Securities, Fair Value Disclosure																				25,421	23,429					0	0					0	0
Derivative Asset, Fair Value, Gross Asset																26,540	4,307
Total																51,961	27,736			25,421	23,429					26,540	4,307					0	0
Derivative Liability, Fair Value, Gross Liability	45	76																																																				45	76
Impairment of Real Estate			(6,000)	[1]	(54,500)	[1]																																							4,500	[1]	50,000	[1]	6,000	1,147	6,000	20,155
Interest Rate Derivative Assets, at Fair Value																				0	0					26,540	4,307					0	0
Notes payable	1,764,437	1,771,891																																															1,764,437		1,764,437		1,771,891
Derivative instruments, at fair value																				$ 0	$ 0					$ (45)	$ 76					$ 0	$ 0
Fair Value Inputs, Cap Rate									8.00%	8.30%	8.50%	8.80%		8.50%	8.80%
Fair Value Inputs, Long-term Revenue Growth Rate							0.00%	(8.30%)					2.50%
Fair Value Inputs, Discount Rate							9.00%	10.50%					10.50%

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