Consolidated Statements of Equity (USD $)	Total	Parent Company [Member] USD ($)	Parent Company [Member] Common Stock USD ($)	Parent Company [Member] Additional Paid-in Capital USD ($)	Parent Company [Member] Accumulated Distributions in Excess of Net Income USD ($)	Parent Company [Member] Accumulated Other Comprehensive Income (Loss) USD ($)	Parent Company [Member] Noncontrolling Interest USD ($)	Parent Company [Member] Redeemable Stock USD ($)	Limited Partner [Member] USD ($)	Total Partnership Capital Limited Partner [Member] USD ($)	Redeemable Stock Limited Partner [Member] USD ($)	General Partners' Capital Account, Class Limited Partner [Member] USD ($)	Accumulated Other Comprehensive Income (Loss) Limited Partner [Member] USD ($)
Beginning balance, Redeemable Stock at Dec. 31, 2010								$ 3,764,000			$ 3,764,000
Beginning balance at Dec. 31, 2010		540,628,000	348,000	1,142,023,000	(575,021,000)	(48,847,000)	22,125,000
Beginning balance (in shares) at Dec. 31, 2010			34,809,000
Partners' Capital, Including Portion Attributable to Noncontrolling Interest									37,079,000	738,157,000		739,657,000	(38,579,000)
Income (Loss), Including Portion Attributable to Noncontrolling Interest									2,580,000
Comprehensive income:
Net income		51,231,000			48,821,000		2,410,000		50,588,000	50,588,000		48,008,000
Other comprehensive income - derivative instruments (cash flow hedges)		13,627,000				12,999,000	628,000			13,688,000			13,688,000
Issuance of units										235,334,000	494,000	235,334,000
Units repurchased and retired										(2,548,000)		(2,548,000)
Exercise of unit options										407,000		407,000
Issuance and registration of common shares (in shares)			3,856,000
General partnership units issued in exchange for limited partnership units									(3,069,000)	0		(3,069,000)
Units reclassified to liabilities										0	(150,000)
Issuance and registration of common shares		235,334,000	38,000	235,296,000				494,000
Shares repurchased and retired (in shares)			(41,000)
Shares repurchased and retired		(2,548,000)	0	(2,548,000)
Exercise of stock options (in shares)			16,000
Exercise of stock options		0	0	407,000					0
Value of Stock Issued During Period, Stock Options Exercised		407,000
Shares issued in exchange for units (in shares)			254,000
Shares issued in exchange for units		0	3,000	3,066,000			(3,069,000)
Shares reclassified to liabilities, shares			0
Shares reclassified to liabilities		0	0	0				(150,000)
Redeemable stock fair market value		71,000			71,000			(71,000)		(71,000)	(71,000)	(71,000)
Adjustment for Noncontrolling Interest Ownership in operating partnership		0		(8,140,000)			8,140,000		7,576,000	(96,000)		(7,672,000)
Amortization of unearned compensation		5,519,000		5,519,000						4,035,000		4,035,000
Distributions									(5,103,000)	(100,807,000)		(95,704,000)
Dividends on common stock ($2.8150, $2.6750 and $2.5425 per share for years ended December 31, 2013, 2012 and 2011)		(95,704,000)			(95,704,000)		0
Dividends on noncontrolling interest units ($2.8150, $2.6750 and $2.5425 per unit for years ended December 31, 2013, 2012 and 2011)		(5,103,000)					(5,103,000)
Ending balance, Redeemable Stock at Dec. 31, 2011								4,037,000			4,037,000
Ending balance at Dec. 31, 2011		743,462,000	389,000	1,375,623,000	(621,833,000)	(35,848,000)	25,131,000
Ending balance (in shares) at Dec. 31, 2011			38,894,000
Comprehensive income:
Net income		25,068,000
Ending balance, Redeemable Stock at Mar. 31, 2012
Beginning balance, Redeemable Stock at Dec. 31, 2011								4,037,000			4,037,000
Beginning balance at Dec. 31, 2011		743,462,000	389,000	1,375,623,000	(621,833,000)	(35,848,000)	25,131,000
Beginning balance (in shares) at Dec. 31, 2011			38,894,000
Partners' Capital, Including Portion Attributable to Noncontrolling Interest									38,154,000	939,007,000		927,734,000	(26,881,000)
Income (Loss), Including Portion Attributable to Noncontrolling Interest									4,697,000
Comprehensive income:
Net income		109,825,000			105,223,000		4,602,000		109,040,000	109,040,000		104,343,000
Other comprehensive income - derivative instruments (cash flow hedges)		11,702,000				9,794,000	1,908,000			11,698,000			11,698,000
Issuance of units										196,326,000	546,000	196,326,000
Units repurchased and retired										(1,990,000)		(1,990,000)
Issuance and registration of common shares (in shares)			3,174,000
General partnership units issued in exchange for limited partnership units									(3,459,000)	0		(3,459,000)
Issuance and registration of common shares		196,326,000	31,000	196,295,000				546,000
Shares repurchased and retired (in shares)			(31,000)
Shares repurchased and retired		(1,990,000)	0	(1,990,000)
Exercise of stock options (in shares)
Exercise of stock options		0	0	0					0
Shares issued in exchange for units (in shares)			206,000
Shares issued in exchange for units		0	2,000	3,457,000			(3,459,000)
Shares reclassified to liabilities, shares			0
Shares reclassified to liabilities		0	0	0				0
Redeemable stock fair market value		(130,000)			(130,000)			130,000		130,000	130,000	130,000
Adjustment for Noncontrolling Interest Ownership in operating partnership		0		(5,587,000)			5,587,000		4,710,000	(9,000)		(4,719,000)
Amortization of unearned compensation		2,233,000		2,233,000						2,233,000		2,233,000
Distributions									(4,873,000)	(116,318,000)		(111,445,000)
Dividends on common stock ($2.8150, $2.6750 and $2.5425 per share for years ended December 31, 2013, 2012 and 2011)		(111,445,000)			(111,445,000)		0
Dividends on noncontrolling interest units ($2.8150, $2.6750 and $2.5425 per unit for years ended December 31, 2013, 2012 and 2011)		(4,873,000)					(4,873,000)
Ending balance, Redeemable Stock at Dec. 31, 2012		4,713,000						4,713,000	4,713,000		4,713,000
Ending balance at Dec. 31, 2012		945,110,000	422,000	1,542,999,000	(603,315,000)	(26,054,000)	31,058,000
Ending balance (in shares) at Dec. 31, 2012			42,243,000
Beginning balance, Redeemable Stock at Sep. 30, 2012
Comprehensive income:
Net income		23,207,000
Ending balance, Redeemable Stock at Dec. 31, 2012		4,713,000
Ending balance at Dec. 31, 2012		945,110,000
Comprehensive income:
Net income		22,005,000
Ending balance, Redeemable Stock at Mar. 31, 2013
Beginning balance, Redeemable Stock at Dec. 31, 2012		4,713,000						4,713,000	4,713,000		4,713,000
Beginning balance at Dec. 31, 2012		945,110,000	422,000	1,542,999,000	(603,315,000)	(26,054,000)	31,058,000
Beginning balance (in shares) at Dec. 31, 2012			42,243,000
Partners' Capital, Including Portion Attributable to Noncontrolling Interest									166,746,000	3,113,518,000		2,946,598,000	174,000
Income (Loss), Including Portion Attributable to Noncontrolling Interest									3,979,000
Comprehensive income:
Net income		119,279,000			115,281,000		3,998,000		107,544,000	107,544,000		103,565,000
Other comprehensive income - derivative instruments (cash flow hedges)		27,054,000				26,162,000	892,000			27,055,000			27,055,000
Issuance of units										2,188,306,000	692,000	2,027,237,000
Units repurchased and retired										(702,000)		(702,000)
Issuance and registration of common shares (in shares)			32,325,000
General partnership units issued in exchange for limited partnership units									(2,519,000)	0		(2,519,000)
Issuance and registration of common shares		2,188,307,000	325,000	2,026,913,000			161,069,000	692,000
Shares repurchased and retired (in shares)			(10,000)
Shares repurchased and retired		(702,000)	0	(702,000)
Exercise of stock options (in shares)	0		111,000
Exercise of stock options		6,212,000	0	6,212,000					6,212,000
Shares issued in exchange for units (in shares)			79,000
Shares issued in exchange for units		0	0	2,519,000			(2,519,000)
Redeemable stock fair market value		355,000			355,000			(355,000)		(355,000)	(355,000)	(355,000)
Correction of classification of equity accounts (Note 1)		0
Correction of classification of equity accounts (Note 1) (Accumulated Distributions in Excess of Net Income)					24,870,000
Correction of classification of equity accounts (Note 1) (Noncontrolling Interest)							2,162,000
Correction of classification of equity accounts (Note 1) (Additional Paid-in Capital)				(27,032,000)
Adjustment for Noncontrolling Interest Ownership in operating partnership		0		19,340,000			(19,340,000)		(25,505,000)	17,819,000		43,324,000
Amortization of unearned compensation		2,268,000		2,268,000						2,268,000		2,268,000
Distributions									(8,432,000)	(174,346,000)		(165,914,000)
Dividends on common stock ($2.8150, $2.6750 and $2.5425 per share for years ended December 31, 2013, 2012 and 2011)		(165,914,000)			(165,914,000)		0
Dividends on noncontrolling interest units ($2.8150, $2.6750 and $2.5425 per unit for years ended December 31, 2013, 2012 and 2011)		(8,432,000)					(8,432,000)
Ending balance, Redeemable Stock at Dec. 31, 2013		5,050,000						5,050,000	5,050,000		5,050,000
Ending balance at Dec. 31, 2013		3,113,537,000	747,000	3,599,549,000	(653,593,000)	108,000	166,726,000
Ending balance (in shares) at Dec. 31, 2013			74,748,000
Beginning balance, Redeemable Stock at Sep. 30, 2013
Comprehensive income:
Net income		(9,810,000)
Ending balance, Redeemable Stock at Dec. 31, 2013		5,050,000
Ending balance at Dec. 31, 2013		$ 3,113,537,000