Shareholders' Equity - Common Shares (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Class of Stock [Line Items]
Net proceeds from the issuance of common shares	$ 969,468,000	$ 58,710,000	$ 44,548,000
Proceeds from Issuance of Common Stock	969,468,000	58,710,000	44,552,000
Common Shares and Common Units [Member]
Class of Stock [Line Items]
Dividends, Common Stock, Cash	247,400,000	229,700,000	226,000,000
Common Stock [Member]
Class of Stock [Line Items]
Common Stock, Ordinary Dividend, Per Share, Cash Paid	$ 1.4312	$ 1.5036	$ 1.4300
Common Stock, Qualified Dividend, Per Share, Cash Paid	$ 0.0000	$ 0.0000	$ 0.0000
Common Stock, Capital gain 15%, Per Share, Cash Paid	$ 0.0000	$ 0.0000	$ 0.1708
Common Stock, Capital gain 20%, Per Share, Cash Paid	$ 0.0016	$ 0.0000	$ 0.0000
Common Stock, IRC SEC 1250 unrecaptured gain 25%, Per Share, Cash Paid	$ 0.4672	$ 0.0164	$ 0.2992
Common Stock, Return of Capital, Per Share, Cash Paid	$ 0.0000	$ 0.3800	$ 0.0000
Common Stock, Dividends, Per Share, Cash Paid	$ 1.9000	$ 1.9000	$ 1.9000
Treasury Stock [Member]
Class of Stock [Line Items]
Stock Repurchase Program, Authorized Amount	100,000,000
Dividend Reinvestment Plan [Member] | Common Stock [Member]
Class of Stock [Line Items]
Stock Issued During Period, Shares, Dividend Reinvestment Plan	1,248,842	1,037,712	1,181,776
Continuous Equity Offering [Member]
Class of Stock [Line Items]
Common stock, Continuous equity offering program, Max amount of equity	200,000,000
Continuous Equity Offering [Member] | Common Stock [Member]
Class of Stock [Line Items]
Stock Issued During Period, Shares, New Issues	1,900,000	0	0
Proceeds from Issuance of Common Stock	$ 75,000,000
Share Repurchase Plan [Member] | Treasury Stock [Member]
Class of Stock [Line Items]
Treasury Shares Repurchased	0	0	0