Total Equity and its Components (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Stockholders Equity Note [Line Items]
Total Equity			$ 743,462	$ 540,628
Net income	29,472	7,680	54,540	16,835
Other comprehensive income - derivative instruments (cash flow hedges)			5,199	6,809
Issuance and registration of common shares			120,150	125,700
Shares repurchased and retired			1,640	2,260
Exercise of stock options				38
Shares issued in exchange for units			0	0
Redeemable stock fair market value			375	235
Adjustment for Noncontrolling Interest Ownership in operating partnership			0	0
Amortization of unearned compensation			1,231	1,686
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)			54,146	46,332
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)			2,458	2,569
Total Equity	865,963	640,300	865,963	640,300
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			20	21
Shares repurchased and retired			0	0
Exercise of stock options				0
Shares issued in exchange for units			1	2
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	410	371	410	371
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			120,130	125,679
Shares repurchased and retired			1,640	2,260
Exercise of stock options				38
Shares issued in exchange for units			2,515	2,876
Redeemable stock fair market value
Adjustment for Noncontrolling Interest Ownership in operating partnership			(3,687)	(5,189)
Amortization of unearned compensation			1,231	1,686
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	1,494,172	1,264,853	1,494,172	1,264,853
Accumulated Distributions in Excess of Net Income
Stockholders Equity Note [Line Items]
Total Equity			(621,833)	(575,021)
Net income			52,050	16,272
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			375	235
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)			54,146	46,332
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	(624,304)	(605,316)	(624,304)	(605,316)
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)			4,957	6,579
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	(30,891)	(42,268)	(30,891)	(42,268)
Noncontrolling Interest
Stockholders Equity Note [Line Items]
Total Equity			25,131	22,125
Net income			2,490	563
Other comprehensive income - derivative instruments (cash flow hedges)			242	230
Issuance and registration of common shares
Shares repurchased and retired
Exercise of stock options
Shares issued in exchange for units			(2,516)	(2,878)
Redeemable stock fair market value
Adjustment for Noncontrolling Interest Ownership in operating partnership			3,687	5,189
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)			2,458	2,569
Total Equity	$ 26,576	$ 22,660	$ 26,576	$ 22,660