Company Quarterly Financial Information - Schedule of Company Quarterly Financial Information (Detail) (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
Interim Reporting [Line Items]
Revenues	$ 86,101	$ 86,374	$ 82,160	$ 80,276	$ 77,749	$ 78,612	$ 75,424	$ 73,531	$ 334,911	$ 305,316	$ 285,138
Net income	18,934	22,322	21,170	21,865	3,920	8,828	9,834	3,013	84,291	25,595	(6,991)
Noncontrolling interests	(81)	(115)	(91)	(65)	(19)	(34)	(88)	12
Dividends to preferred shareholders	(922)	(922)	(922)	(922)	(922)	(922)	(922)	(1,689)	(3,688)	(4,455)	(7,503)
Preferred stock redemption costs								(1,757)		(1,757)	(44)
Net income (loss) available to common shareholders	$ 17,931	$ 21,285	$ 20,157	$ 20,878	$ 2,979	$ 7,872	$ 8,824	$ (421)	$ 80,251	$ 19,254	$ (14,507)
Net income (loss) available to common shareholders/unitholders - basic	$ 0.33	$ 0.39	$ 0.37	$ 0.39	$ 0.06	$ 0.15	$ 0.18	$ (0.01)	$ 1.49	$ 0.38	$ (0.30)
Net income (loss) available to common shareholders/unitholders - diluted	$ 0.33	$ 0.39	$ 0.37	$ 0.39	$ 0.06	$ 0.15	$ 0.17	$ (0.01)	$ 1.48	$ 0.38	$ (0.30)