Shareholders' Equity - Common Shares (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Class of Stock [Line Items]
Authorized but Unissued Preferred Shares	14,723,000
Net proceeds from the issuance of common shares	$ 58,710,000	$ 44,548,000	$ 37,416,000
Common Shares and Common Units [Member]
Class of Stock [Line Items]
Dividends, Common Stock, Cash	229,700,000	226,000,000	222,600,000
Common Stock [Member]
Class of Stock [Line Items]
Common Stock, Ordinary Dividend, Per Share, Cash Paid	$ 1.5036	$ 1.4300	$ 1.4812
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	$ 0.1708	$ 0.0000
Common Stock, IRC SEC 1250 unrecaptured gain 25%, Per Share, Cash Paid	$ 0.0164	$ 0.2992	$ 0.0128
Common Stock, Return of Capital, Per Share, Cash Paid	$ 0.3800	$ 0.0000	$ 0.4060
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	1,037,712	1,181,776	915,363
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] | Liberty Property Limited Partnership [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, Other	0	0	0
Continuous Equity Offering [Member] | Common Stock [Member] | Liberty Property Limited Partnership [Member]
Class of Stock [Line Items]
Stock Issued During Period, Shares, Other	0	0	0
Share Repurchase Plan [Member] | Treasury Stock [Member]
Class of Stock [Line Items]
Treasury Shares Repurchased	0	0	0