Mortgages and Notes Receivable (Details) (USD $)			12 Months Ended			9 Months Ended		3 Months Ended		9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Dec. 31, 2010 Seller Financing [Member] transactions	Sep. 30, 2013 Seller Financing [Member]	Dec. 31, 2012 Seller Financing [Member]	Sep. 30, 2013 Mortgage Receivable [Member]	Dec. 31, 2012 Mortgage Receivable [Member]	Sep. 30, 2013 Promissory Notes [Member]	Sep. 30, 2012 Promissory Notes [Member]	Sep. 30, 2013 Promissory Notes [Member]	Sep. 30, 2012 Promissory Notes [Member]	Dec. 31, 2012 Promissory Notes [Member]
Schedule of mortgages and notes receivable [Abstract]
Mortgages and notes receivable, gross				$ 16,454,000	$ 15,853,000	$ 9,207,000	$ 8,648,000	$ 970,000		$ 970,000		$ 1,153,000
Mortgages and notes receivable, allowance	(340,000)	(182,000)		0	0	0		(340,000)	(211,000)	(340,000)	(211,000)
Mortgages and notes receivable, net	26,291,000	25,472,000		16,454,000	15,853,000	9,207,000	8,648,000	630,000		630,000		971,000
Number of transactions with seller financing			2
Additional loan commitment to third party							8,400,000
Cumulative funds for future infrastructure disbursed to third party						200,000
Notes receivable allowance, promissory notes [Roll Forward]
Beginning notes receivable allowance	340,000	182,000		0	0	0		376,000	118,000	182,000	61,000
Recoveries/write-offs/other								(36,000)	93,000	158,000	150,000
Total notes receivable allowance	$ 340,000	$ 182,000		$ 0	$ 0	$ 0		$ 340,000	$ 211,000	$ 340,000	$ 211,000