Total Equity and its Components (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Stockholders Equity Note [Line Items]
Total Equity	$ 743,462	$ 540,628
Net income	25,068	9,155
Other comprehensive income - derivative instruments (cash flow hedges)	4,248	7,461
Issuance and registration of common shares	120,143	91,417
Shares repurchased and retired	1,415	1,939
Exercise of stock options		26
Shares issued in exchange for units	0	0
Redeemable stock fair market value	293	43
Adjustment for Noncontrolling Interest Ownership in operating partnership	0	0
Amortization of unearned compensation	814	844
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)	27,030	22,971
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)	1,279	1,310
Total Equity	863,718	623,268
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	15
Shares repurchased and retired	0	0
Exercise of stock options		0
Shares issued in exchange for units	0	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	409	365
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	120,123	91,402
Shares repurchased and retired	1,415	1,939
Exercise of stock options		26
Shares issued in exchange for units	28	2,069
Redeemable stock fair market value
Adjustment for Noncontrolling Interest Ownership in operating partnership	(3,990)	(3,955)
Amortization of unearned compensation	814	844
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	1,491,183	1,230,470
Accumulated Distributions in Excess of Net Income
Stockholders Equity Note [Line Items]
Total Equity	(621,833)	(575,021)
Net income	23,890	8,844
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	293	43
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)	27,030	22,971
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	(625,266)	(589,191)
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)	4,048	7,208
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	(31,800)	(41,639)
Noncontrolling Interest
Stockholders Equity Note [Line Items]
Total Equity	25,131	22,125
Net income	1,178	311
Other comprehensive income - derivative instruments (cash flow hedges)	200	253
Issuance and registration of common shares
Shares repurchased and retired
Exercise of stock options
Shares issued in exchange for units	(28)	(2,071)
Redeemable stock fair market value
Adjustment for Noncontrolling Interest Ownership in operating partnership	3,990	3,955
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)	1,279	1,310
Total Equity	$ 29,192	$ 23,263