Consolidated Statements of Equity (Unaudited) (USD $) In Thousands, unless otherwise specified	Total	Redeemable Noncontrolling Partnership Interests [Member]	Preferred Stock [Member]	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Other Comprehensive Income [Member]	Dividends in Excess of Cumulative Earnings [Member]	Noncontrolling Interest [Member]	Total Shareholder Equity [Member]
Balance, redeemable noncontrolling partnership interests at Dec. 31, 2011		$ 32,271
Balance at Dec. 31, 2011	1,470,391		23	1,484	1,657,927	3,425	(399,581)	207,113	1,263,278
Net income	30,318						26,049	4,269	26,049
Other Comprehensive Income	1,788	14				1,407		381	1,407
Partners' Capital Account, Redemptions	(6,359)							(6,359)
Dividends declared - common stock	(32,700)						(32,700)		(32,700)
Dividends declared - preferred stock	(10,594)						(10,594)		(10,594)
Issuance of common stock and restricted common stock	284			2	282				284
Cancellation of restricted common stock	(247)				(247)				(247)
Exercise of stock options	2,338			1	2,337				2,338
Accrual under deferred compensation arrangements	14				14				14
Amortization of deferred compensation	1,041				1,041				1,041
Distributions to noncontrolling interests	(9,454)	(1,893)						(9,454)
Adjustment for noncontrolling interests	(843)	843			1,811			(2,654)	1,811
Adjustment to record redeemable noncontrolling interests at redemption value	(4,272)	4,272			(4,272)				(4,272)
Redeemable Noncontrolling Partnership Interests [Roll forward]
Net income		1,089
Balance, redeemable noncontrolling partnership interests at Mar. 31, 2012		36,596
Balance at Mar. 31, 2012	1,441,705		23	1,487	1,658,893	4,832	(416,826)	193,296	1,248,409
Balance, redeemable noncontrolling partnership interests at Dec. 31, 2012		40,248
Balance at Dec. 31, 2012	1,521,097		25	1,613	1,773,630	6,986	(453,561)	192,404	1,328,693
Net income	34,011						30,313	3,698	30,313
Other Comprehensive Income	1,030	10				864		166	864
Issuance of common stock	43,869			19	43,850				43,869
Dividends declared - common stock	(37,713)						(37,713)		(37,713)
Dividends declared - preferred stock	(11,223)						(11,223)		(11,223)
Issuance of common stock and restricted common stock	0			2	(2)				0
Cancellation of restricted common stock	(402)				(402)				(402)
Amortization of deferred compensation	1,464				1,464				1,464
Distributions to noncontrolling interests	(8,544)	(1,744)						(8,544)
Adjustment for noncontrolling interests	(157)	157			(10,909)			10,752	(10,909)
Adjustment to record redeemable noncontrolling interests at redemption value	(4,156)	4,156			(3,523)			(633)	(3,523)
Redeemable Noncontrolling Partnership Interests [Roll forward]
Net income		788
Balance, redeemable noncontrolling partnership interests at Mar. 31, 2013		43,615
Balance at Mar. 31, 2013	$ 1,539,276		$ 25	$ 1,634	$ 1,804,108	$ 7,850	$ (472,184)	$ 197,843	$ 1,341,433