SUPPLEMENTARY QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								9 Months Ended	12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Sep. 30, 2011	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
SUPPLEMENTARY QUARTERLY FINANCIAL DATA
Revenues	$ 93,212	$ 84,097	$ 78,040	$ 74,481	$ 73,154	$ 71,979	$ 68,777	$ 67,587		$ 329,830	$ 281,497	$ 280,146
Cost of operations	58,833	52,882	52,188	50,451	49,569	48,418	45,971	46,724		214,354	190,682	208,259
Revenue less cost of operations	34,379	31,215	25,852	24,030	23,585	23,561	22,806	20,863		115,476	90,815	71,887
Net income	18,414	17,352	12,517	10,140	10,788	9,482	7,925	5,179		58,423	33,374	39,093
Net income attributable to common stockholders	16,278	15,261	10,609	8,301	8,916	7,667	6,180	3,568		50,449	26,331	31,977
Net income - basic (in dollars per share)	$ 0.17	$ 0.16	$ 0.12	$ 0.09	$ 0.10	$ 0.09	$ 0.07	$ 0.04		$ 0.55	$ 0.30	$ 0.37
Net income - diluted (in dollars per share)	$ 0.17	$ 0.16	$ 0.12	$ 0.09	$ 0.10	$ 0.09	$ 0.07	$ 0.04		$ 0.54	$ 0.30	$ 0.37
Cumulative effect of prior year misstatement corrected in current year
Asset management fee									203
Cumulative adjustment for unrecorded asset management fees
Cumulative effect of prior year misstatement corrected in current year
Asset management fee										4,425
Adjustment to reduce equity in earnings										$ 1,157