Shareholders' Equity - Common Shares (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Continuous Equity Offering [Abstract]
Common stock, Continuous equity offering program, Max amount of equity	$ 200,000,000
Proceeds from Issuance of Common Stock	58,473,000	969,468,000	58,710,000
Common Shares and Common Units [Member]
Common Shares [Abstract]
Dividends, Common Stock, Cash	286,900,000	247,400,000	229,700,000
Common Stock [Member]
Common Shares [Abstract]
Common Stock, Ordinary Dividend, Per Share, Cash Paid	$ 1.2556	$ 1.4312	$ 1.5036
Common Stock, Capital gain 20%, Per Share, Cash Paid	$ 0.0000	$ 0.0016	$ 0.0000
Common Stock, IRC SEC 1250 unrecaptured gain 25%, Per Share, Cash Paid	$ 0.3712	$ 0.4672	$ 0.0164
Common Stock, Return of Capital, Per Share, Cash Paid	$ 0.2732	$ 0.0000	$ 0.3800
Common Stock, Dividends, Per Share, Cash Paid	$ 1.9000	$ 1.9000	$ 1.9000
Dividend Reinvestment and Share Purchase Plan [Abstract]
Stock Issued During Period, Shares, Dividend Reinvestment Plan	1,643,536	1,248,842	1,037,712
Continuous Equity Offering [Abstract]
Number of common shares issued	0	1,900,000	0
Proceeds from Issuance of Common Stock		$ 75,000,000