Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified			12 Months Ended																								6 Months Ended			6 Months Ended				12 Months Ended				3 Months Ended		6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	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]	Jun. 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]	Dec. 31, 2012 Estimate of Fair Value Measurement [Member] Fair Value, Measurements, Nonrecurring [Member]	Jun. 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]	Dec. 31, 2012 Fair Value, Inputs, Level 1 [Member] Fair Value, Measurements, Nonrecurring [Member]	Jun. 30, 2013 Fair Value, Inputs, Level 2 [Member]	Dec. 31, 2012 Fair Value, Inputs, Level 2 [Member]	Jun. 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]	Dec. 31, 2012 Fair Value, Inputs, Level 2 [Member] Fair Value, Measurements, Nonrecurring [Member]	Jun. 30, 2013 Fair Value, Inputs, Level 3 [Member]	Dec. 31, 2012 Fair Value, Inputs, Level 3 [Member]	Jun. 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]	Dec. 31, 2012 Fair Value, Inputs, Level 3 [Member] Fair Value, Measurements, Nonrecurring [Member]	Jun. 30, 2013 Unsecured Credit Facilities [Member]	Jun. 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]	Jun. 30, 2013 Notes Payable, Other Payables [Member] Minimum	Jun. 30, 2013 Notes Payable, Other Payables [Member] Maximum	Jun. 30, 2013 Notes Receivable [Member] Minimum	Jun. 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]		Jun. 30, 2013 Partnership Interest [Member]	Jun. 30, 2012 Partnership Interest [Member]	Jun. 30, 2013 Partnership Interest [Member]	Jun. 30, 2012 Partnership Interest [Member]	Dec. 31, 2012 Partnership Interest [Member]	Jun. 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]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes receivable	$ 18,502	$ 23,751																																				$ 18,502		$ 18,502		$ 23,751
Long-Lived Assets													49,673			0					0					49,673
Risk Free Interest Rate																											1.60%			2.70%	3.65%	7.07%	7.69%
Notes Receivable, Fair Value																						18,153	23,677
Trading securities held in trust, at fair value	24,457	23,429																																				24,457		24,457		23,429
Notes Payable, Fair Value																	1,924,578	2,000,000
Unsecured credit facilities	125,000	170,000																																				125,000		125,000		170,000
Debt Instrument, Fair Value Disclosure																												125,240	170,200
Trading Securities, Fair Value Disclosure														24,457	23,429				0	0				0	0
Derivative Asset, Fair Value, Gross Asset											26,011	4,307
Total											50,468	27,736		24,457	23,429				26,011	4,307				0	0
Derivative Liability, Fair Value, Gross Liability	51	76																																									51	76
Impairment of Real Estate			(54,500)	[1]																														4,500	[1]	50,000	[1]	0	19,008	0	19,008
Interest Rate Derivative Assets, at Fair Value														0	0				26,011	4,307				0	0
Notes payable	1,767,124	1,771,891																																				1,767,124		1,767,124		1,771,891
Derivative instruments, at fair value														$ 0	$ 0				$ (51)	$ 76				$ 0	$ 0
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.