Income Taxes (Reconciliation of Net Income to REIT Taxable Income) (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Income Tax Disclosure [Abstract]
Income tax reconciliation, Estimated designated dividends included in Dividends Paid Deduction									$ 84,000,000
Net income attributable to common shareholders	178,498,000	38,283,000	35,272,000	40,036,000	129,996,000	70,720,000	72,172,000	63,476,000	292,089,000	[1]	336,364,000	[1]	283,390,000
(Income) loss from taxable Real Estate Investment Trust subsidiaries included above									(1,523,000)		(2,940,000)		3,323,000
Net income from Real Estate Investment Trust operations									290,566,000		333,424,000		286,713,000
Book depreciation and amortization, including discontinued operations									238,989,000		223,198,000		213,479,000
Tax depreciation and amortization									(200,153,000)		(204,059,000)		(171,060,000)
Book/tax difference on gains/losses from capital transactions									(35,635,000)		(86,358,000)		(63,832,000)
Other book/tax differences, net									8,805,000		(9,427,000)		(40,961,000)
Real Estate Investment Trust Taxable Income									302,572,000		256,778,000		224,339,000
Dividends paid deduction									(302,572,000)	[2]	(256,778,000)	[3]	(224,339,000)	[4]
Dividends Paid In Excess Of Taxable Income									0		0		0
Income tax reconciliation, designated dividends included in dividends paid deduction											$ 5,100,000		$ 26,600,000

[1] Net income per share is computed independently for each of the quarters presented. Therefore, the sum of quarterly net income per share amounts may not equal the total computed for the year.

[2] The dividends paid deduction includes estimated designated dividends from 2015 of approximately $84.0 million.

[3] We borrowed approximately $5.1 million from 2014 for designated dividends in 2013.

[4] We borrowed approximately $26.6 million from 2013 for designated dividends in 2012.