Quarterly Results of Operations (Unaudited) (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 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		Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Quarterly Financial Information [Line Items]
Total revenue	$ 251,439	[1],[2]	$ 247,557	[1],[2]	$ 239,304	[1],[2]	$ 230,411	[1],[2]	$ 226,552	[1],[2]	$ 222,224	[1],[2]	$ 215,029	[1],[2]	$ 210,196	[1],[2]	$ 968,711	$ 874,005	$ 829,924
Income (loss) from continuing operations	51,810	[1],[2],[3]	45,709	[1],[2],[3]	36,609	[1],[2],[3]	31,814	[1],[2],[3]	26,202	[1],[2]	25,581	[1],[2]	30,313	[1],[2]	21,678	[1],[2]
Total discontinued operations	271,155	[1],[2]	(1,031)	[1],[2]	6,583	[1],[2]	(1,278)	[1],[2]	466	[1],[2]	(1,601)	[1],[2]	20,753	[1],[2]	50,688	[1],[2]	275,432	70,306	71,576
Net income attributable to common stockholders	$ 323,085	[1]	$ 44,824	[1]	$ 43,373	[1]	$ 30,341	[1]	$ 27,030	[1]	$ 24,654	[1]	$ 51,125	[1]	$ 72,523	[1]	$ 441,622	$ 175,331	$ 155,647
Net income per common share - basic	$ 3.40	[1]	$ 0.49	[1]	$ 0.50	[1]	$ 0.35	[1]	$ 0.32	[1]	$ 0.29	[1]	$ 0.61	[1]	$ 0.89	[1]	$ 4.89	$ 2.08	$ 1.94
Net income per common share - diluted	$ 3.38	[1]	$ 0.49	[1]	$ 0.49	[1]	$ 0.35	[1]	$ 0.31	[1]	$ 0.29	[1]	$ 0.61	[1]	$ 0.88	[1]	$ 4.87	$ 2.07	$ 1.93

[1] Amounts may not equal full year results due to rounding.

[2] Amounts may not equal previously reported results due to reclassification between income from continuing operations and income from discontinued operations.

[3] Income from continuing operations for the third quarter of 2011 includes an impairment charge of $14,052 associated with the change in the Company's intent on two land parcels and the write down of an investment in an unconsolidated joint venture.