Unaudited Quarterly Operating Results (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Selected Quarterly Financial Information [Abstract]
Total revenues	$ 13,904	[1]	$ 16,442	[1]	$ 8,177	[1]	$ 9,921	[1]	$ 20,169	[1]	$ 15,099	[1]	$ 6,219	[1]	$ 22,969	[1],[2]	$ 48,444	[1]	$ 64,456	[1]	$ 36,553
Segment profit (loss)																	3,128		20,874		5,239
Net income (loss) attributable to common stockholders	27		4,021		118		275		5,238		2,558		(638)		8,736	[2]	4,441		15,894		4,175
Net income per share attributable to common stockholders, diluted	$ 0.00	[3]	$ 0.20	[3]	$ 0.01	[3]	$ 0.01	[3]	$ 0.26	[3]	$ 0.13	[3]	$ (0.03)	[3]	$ 0.44	[2],[3]	$ 0.22		$ 0.80		$ 0.22
Revenue from sale of conservation easements															15,750
Reportable Segment [Member]
Selected Quarterly Financial Information [Abstract]
Segment profit (loss)	$ 3,851		$ 6,592		$ 2,799		$ 3,158		$ 10,196		$ 5,690		$ 989		$ 16,276	[2]	$ 16,400	[4]	$ 33,151	[4]	$ 10,851	[4]

[1] Includes investment income and other income.

[2] Includes revenue of $15,750,000 from the sale of conservation easements.

[3] Net income (loss) per share on a diluted basis. Quarterly rounding of per share amounts can result in a variance from the reported annual amount.

[4] Segment profits are revenues from operations less operating expenses, excluding investment income and expense, corporate expenses, equity in earnings of unconsolidated joint ventures, and income taxes.