Total Equity and its Components (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Stockholders Equity Note [Line Items]
Total Equity			$ 743,462	$ 540,628
Net income	32,078	14,451	86,618	31,286
Other comprehensive income - derivative instruments (cash flow hedges)			7,111	7,527
Issuance and registration of common shares			173,962	170,106
Shares repurchased and retired			1,863	2,471
Exercise of stock options				407
Shares issued in exchange for units			0	0
Redeemable stock fair market value			168	(223)
Adjustment for Noncontrolling Interest Ownership in operating partnership			0	0
Amortization of unearned compensation			1,730	4,445
Dividends on common stock ($0.6600 and $0.6275 per share for the three month periods ended March 31, 2012 and March 31, 2011 respectively)			81,813	70,027
Dividends on noncontrolling interest units ($0.6600 and $0.6275 per share for the thre month periods ended March 31, 2012 and March 31, 2011 respectively)			3,667	3,825
Total Equity	925,372	678,299	925,372	678,299
Common Stock
Stockholders Equity Note [Line Items]
Total Equity			389	348
Net income
Other comprehensive income - derivative instruments (cash flow hedges)
Issuance and registration of common shares			28	27
Shares repurchased and retired			0	0
Exercise of stock options				0
Shares issued in exchange for units			2	3
Redeemable stock fair market value
Adjustment for Noncontrolling Interest Ownership in operating partnership
Amortization of unearned compensation
Dividends on common stock ($0.6600 and $0.6275 per share for the three month periods ended March 31, 2012 and March 31, 2011 respectively)
Dividends on noncontrolling interest units ($0.6600 and $0.6275 per share for the thre month periods ended March 31, 2012 and March 31, 2011 respectively)
Total Equity	419	378	419	378
Additional Paid-in Capital
Stockholders Equity Note [Line Items]
Total Equity			1,375,623	1,142,023
Net income
Other comprehensive income - derivative instruments (cash flow hedges)
Issuance and registration of common shares			173,934	170,079
Shares repurchased and retired			1,863	2,471
Exercise of stock options				407
Shares issued in exchange for units			2,670	3,001
Redeemable stock fair market value
Adjustment for Noncontrolling Interest Ownership in operating partnership			(4,812)	(7,015)
Amortization of unearned compensation			1,730	4,445
Dividends on common stock ($0.6600 and $0.6275 per share for the three month periods ended March 31, 2012 and March 31, 2011 respectively)
Dividends on noncontrolling interest units ($0.6600 and $0.6275 per share for the thre month periods ended March 31, 2012 and March 31, 2011 respectively)
Total Equity	1,520,250	1,310,469	1,520,250	1,310,469
Accumulated Distributions in Excess of Net Income
Stockholders Equity Note [Line Items]
Total Equity			(621,833)	(575,021)
Net income			82,916	30,063
Other comprehensive income - derivative instruments (cash flow hedges)
Issuance and registration of common shares
Shares repurchased and retired
Exercise of stock options
Shares issued in exchange for units
Redeemable stock fair market value			168	(223)
Adjustment for Noncontrolling Interest Ownership in operating partnership
Amortization of unearned compensation
Dividends on common stock ($0.6600 and $0.6275 per share for the three month periods ended March 31, 2012 and March 31, 2011 respectively)			81,813	70,027
Dividends on noncontrolling interest units ($0.6600 and $0.6275 per share for the thre month periods ended March 31, 2012 and March 31, 2011 respectively)
Total Equity	(596,027)	(614,762)	(596,027)	(614,762)
Accumulated Other Comprehensive Income (Loss)
Stockholders Equity Note [Line Items]
Total Equity			(35,848)	(48,847)
Net income
Other comprehensive income - derivative instruments (cash flow hedges)			5,381	7,263
Issuance and registration of common shares
Shares repurchased and retired
Exercise of stock options
Shares issued in exchange for units
Redeemable stock fair market value
Adjustment for Noncontrolling Interest Ownership in operating partnership
Amortization of unearned compensation
Dividends on common stock ($0.6600 and $0.6275 per share for the three month periods ended March 31, 2012 and March 31, 2011 respectively)
Dividends on noncontrolling interest units ($0.6600 and $0.6275 per share for the thre month periods ended March 31, 2012 and March 31, 2011 respectively)
Total Equity	(30,467)	(41,584)	(30,467)	(41,584)
Noncontrolling Interest [Member]
Stockholders Equity Note [Line Items]
Total Equity			25,131	22,125
Net income			3,702	1,223
Other comprehensive income - derivative instruments (cash flow hedges)			1,730	264
Issuance and registration of common shares
Shares repurchased and retired
Exercise of stock options
Shares issued in exchange for units			(2,672)	(3,004)
Redeemable stock fair market value
Adjustment for Noncontrolling Interest Ownership in operating partnership			4,812	7,015
Amortization of unearned compensation
Dividends on common stock ($0.6600 and $0.6275 per share for the three month periods ended March 31, 2012 and March 31, 2011 respectively)			0	0
Dividends on noncontrolling interest units ($0.6600 and $0.6275 per share for the thre month periods ended March 31, 2012 and March 31, 2011 respectively)			3,667	3,825
Total Equity	$ 31,197	$ 23,798	$ 31,197	$ 23,798
Noncontrolling Interest [Member]
Stockholders Equity Note [Line Items]
Error Corrections and Prior Period Adjustments, Description			0