Total Equity and its Components (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended																						3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013 Common Stock [Member]	Dec. 31, 2012 Common Stock [Member]	Mar. 31, 2012 Common Stock [Member]	Dec. 31, 2011 Common Stock [Member]	Mar. 31, 2013 Additional Paid-in Capital [Member]	Dec. 31, 2012 Additional Paid-in Capital [Member]	Mar. 31, 2012 Additional Paid-in Capital [Member]	Dec. 31, 2011 Additional Paid-in Capital [Member]	Mar. 31, 2013 Accumulated Distributions in Excess of Net Income	Dec. 31, 2012 Accumulated Distributions in Excess of Net Income	Mar. 31, 2012 Accumulated Distributions in Excess of Net Income	Dec. 31, 2011 Accumulated Distributions in Excess of Net Income	Mar. 31, 2013 Accumulated Other Comprehensive Income (Loss) [Member]	Dec. 31, 2012 Accumulated Other Comprehensive Income (Loss) [Member]	Mar. 31, 2012 Accumulated Other Comprehensive Income (Loss) [Member]	Dec. 31, 2011 Accumulated Other Comprehensive Income (Loss) [Member]	Mar. 31, 2013 Noncontrolling Interest [Member]	Dec. 31, 2012 Noncontrolling Interest [Member]	Mar. 31, 2012 Noncontrolling Interest [Member]	Dec. 31, 2011 Noncontrolling Interest [Member]	Mar. 31, 2013 Additional Paid-in Capital [Member]	Mar. 31, 2012 Additional Paid-in Capital [Member]	Mar. 31, 2013 Common Stock [Member]	Mar. 31, 2012 Common Stock [Member]	Mar. 31, 2013 Accumulated Other Comprehensive Income (Loss) [Member]	Mar. 31, 2012 Accumulated Other Comprehensive Income (Loss) [Member]	Mar. 31, 2013 Noncontrolling Interest [Member]	Mar. 31, 2012 Noncontrolling Interest [Member]	Mar. 31, 2013 Accumulated Distributions in Excess of Net Income	Mar. 31, 2012 Accumulated Distributions in Excess of Net Income
Stockholders Equity Note [Line Items]
Total Equity	$ 945,110	$ 743,462	$ 426	$ 422	$ 409	$ 389	$ 1,565,755	$ 1,542,999	$ 1,491,183	$ 1,375,623	$ (612,128)	$ (603,315)	$ (625,266)	$ (621,833)	$ (21,869)	$ (26,054)	$ (31,800)	$ (35,848)	$ 30,129	$ 31,058	$ 29,192	$ 25,131
Net income	22,005	25,068																											825	1,178
Net Income (Loss) Attributable to Parent	21,180	23,890																													21,180	23,890
Other comprehensive income - derivative instruments (cash flow hedges)	4,363	4,248																									4,185	4,048	178	200
Issuance and registration of common shares	22,058	120,143																					22,055	120,123	3	20
Shares repurchased and retired	673	1,415																					673	1,415	0	0
Shares issued in exchange for units	0	0																					(442)	(28)	(1)	0			(443)	(28)
Redeemable stock fair market value	319	293																													319	293
Adjustment for Noncontrolling Interest Ownership in operating partnership	0	0																					(302)	(3,990)					(302)	(3,990)
Amortization of unearned compensation	630	814																					630	814
Total Equity	962,313	863,718	426	422	409	389	1,565,755	1,542,999	1,491,183	1,375,623	(612,128)	(603,315)	(625,266)	(621,833)	(21,869)	(26,054)	(31,800)	(35,848)	30,129	31,058	29,192	25,131
Dividends on common stock ($.6950 and $.6600 for the 3 months ended March 31, 2013 and 2012)	29,674	27,030																											0	0	29,674	27,030
Dividends on noncontrolling interest units ($.6950 and $.6600 for the 3 months ended March 31, 2013 and 2012)	$ 1,187	$ 1,279																											$ 1,187	$ 1,279