Consolidated Statements of Equity (USD $) In Thousands, except Share data, unless otherwise specified	Total	Preferred Stock	Common Stock	Additional Paid-In Capital	Accumulated Distribution in Excess of Net Income	Accumulated Other Comprehensive Income (Loss)	Noncontrolling Interest	Redeemable Stock
Beginning balance at Dec. 31, 2009	$ 453,226	$ 62	$ 290	$ 988,642	$ (510,993)	$ (47,435)	$ 22,660
Beginning balance, Redeemable Stock at Dec. 31, 2009								2,802
Beginning balance (in shares) at Dec. 31, 2009		6,200,000	29,037,000
Comprehensive income:
Net income	30,875				29,761		1,114
Other comprehensive income - derivative instruments (cash flow hedges)	(1,518)					(1,412)	(106)
Issuance and registration of common shares (in shares)			5,673,000
Issuance and registration of common shares	305,344		57	305,287				387
Shares repurchased and retired (in shares)			(22,000)
Shares repurchased and retired	(1,175)		0	(1,175)
Exercise of stock options (in shares)			7,000
Exercise of stock options	173		0	173
Shares issued in exchange for units (in shares)			114,000
Shares issued in exchange for units	1		1	1,266			(1,266)
shares reclassified to liabilities, shares			0
Shares reclassified to liabilities	0		0	0				(269)
Redeemable stock fair market value	(844)				(844)			844
Adjustment for Noncontrolling Interest Ownership in operating partnership	0			(5,364)			5,364
Amortization of unearned compensation	3,005			3,005
Dividends on common stock ($2.6750, $2.5425 and $2.4725 per share for years ended December 31, 2012, 2011 and 2010)	(81,247)				(81,247)		0
Dividends on noncontrolling interest units ($2.6750, $2.5425 and $2.4725 per unit for years ended December 31, 2012, 2011 and 2010)	(5,641)						(5,641)
Redemption of preferred stock (in shares)		(6,200,000)
Redemption of preferred stock	(155,022)	(62)		(149,811)	(5,149)
Dividends on preferred stock	(6,549)				(6,549)
Ending balance at Dec. 31, 2010	540,628	0	348	1,142,023	(575,021)	(48,847)	22,125
Ending balance, Redeemable Stock at Dec. 31, 2010								3,764
Ending balance (in shares) at Dec. 31, 2010		0	34,809,000
Comprehensive income:
Net income	51,231				48,821		2,410
Other comprehensive income - derivative instruments (cash flow hedges)	13,627					12,999	628
Issuance and registration of common shares (in shares)			3,856,000
Issuance and registration of common shares	235,334		38	235,296				494
Shares repurchased and retired (in shares)			(41,000)
Shares repurchased and retired	(2,548)		0	(2,548)
Exercise of stock options (in shares)			16,000
Exercise of stock options	407		0	407
Shares issued in exchange for units (in shares)			254,000
Shares issued in exchange for units	0		3	3,066			(3,069)
shares reclassified to liabilities, shares			0
Shares reclassified to liabilities	0		0	0				(150)
Redeemable stock fair market value	71				71			(71)
Adjustment for Noncontrolling Interest Ownership in operating partnership	0			(8,140)			8,140
Amortization of unearned compensation	5,519			5,519
Dividends on common stock ($2.6750, $2.5425 and $2.4725 per share for years ended December 31, 2012, 2011 and 2010)	(95,704)				(95,704)		0
Dividends on noncontrolling interest units ($2.6750, $2.5425 and $2.4725 per unit for years ended December 31, 2012, 2011 and 2010)	(5,103)						(5,103)
Dividends on preferred stock	0
Ending balance at Dec. 31, 2011	743,462	0	389	1,375,623	(621,833)	(35,848)	25,131
Ending balance, Redeemable Stock at Dec. 31, 2011	4,037							4,037
Ending balance (in shares) at Dec. 31, 2011		0	38,894,000
Comprehensive income:
Net income	109,825				105,223		4,602
Other comprehensive income - derivative instruments (cash flow hedges)	11,702					9,794	1,908
Issuance and registration of common shares (in shares)			3,174,000
Issuance and registration of common shares	196,326		31	196,295				546
Shares repurchased and retired (in shares)			(31,000)
Shares repurchased and retired	(1,990)		0	(1,990)
Exercise of stock options (in shares)	0
Shares issued in exchange for units (in shares)			206,000
Shares issued in exchange for units	0		2	3,457			(3,459)
Redeemable stock fair market value	(130)				(130)			130
Adjustment for Noncontrolling Interest Ownership in operating partnership	0			(5,587)			5,587
Correction of classification of equity accounts (Note 1)	0			(27,032)	24,870		2,162
Amortization of unearned compensation	2,233			2,233
Dividends on common stock ($2.6750, $2.5425 and $2.4725 per share for years ended December 31, 2012, 2011 and 2010)	(111,445)				(111,445)		0
Dividends on noncontrolling interest units ($2.6750, $2.5425 and $2.4725 per unit for years ended December 31, 2012, 2011 and 2010)	(4,873)						(4,873)
Dividends on preferred stock	0
Ending balance at Dec. 31, 2012	945,110	0	422	1,542,999	(603,315)	(26,054)	31,058
Ending balance, Redeemable Stock at Dec. 31, 2012	$ 4,713							$ 4,713
Ending balance (in shares) at Dec. 31, 2012		0	42,243,000