Unaudited Quarterly Operating Results (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended																	12 Months Ended
	Dec. 31, 2013		Sep. 30, 2013		Jun. 30, 2013		Mar. 31, 2013		Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Mar. 31, 2011	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Selected Quarterly Financial Information [Abstract]
Total revenues	$ 13,216	[1]	$ 15,364	[1]	$ 7,727	[1]	$ 10,038	[1]	$ 13,904	[1]	$ 16,442	[1]	$ 8,177	[1]	$ 9,921	[1]		$ 46,345	[1]	$ 48,444	[1]
Net income (loss) attributable to common stockholders	(826)		2,292		2,084		615		27		4,021		118		275			4,165		4,441		15,894
Net income per share attributable to common stockholders, diluted, in dollars per share	$ (0.04)	[2]	$ 0.11	[2]	$ 0.10	[2]	$ 0.04	[2]	$ 0.00	[2]	$ 0.20	[2]	$ 0.01	[2]	$ 0.01	[2]		$ 0.20		$ 0.22		$ 0.80
Revenue from sale of conservation easements																	15,750
Reportable Segment
Selected Quarterly Financial Information [Abstract]
Total revenues																		50,351		50,979		65,372
Segment profit (loss)	$ 3,413		$ 4,658		$ 1,825		$ 3,921		$ 3,851		$ 6,592		$ 2,799		$ 3,158			$ 13,817		$ 16,400

[1] Includes investment income and other income.

[2] 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.