Common Share Capitalization (Narrative) (Details) (USD $)	0 Months Ended		12 Months Ended			29 Months Ended		0 Months Ended
	May 14, 2013	Aug. 04, 2011	Dec. 31, 2013 Dividend	Dec. 31, 2012 Dividend	Dec. 31, 2011 Dividend	Dec. 31, 2013	Sep. 12, 2012	Aug. 20, 2012 Secondary public offering [Member]	Sep. 12, 2012 Secondary public offering overallotment option [Member]
Common Stock Capitalization [Line Items]
Shares issued	5,000,000		5,000,000	4,025,000	0			3,500,000	525,000
Overallotment Option Percentage By Underwriters							15.00%
Proceeds From Issuance Of Common Shares, Net	$ 125,300,000							$ 76,700,000	$ 11,100,000
Common Shares Issued And Outstanding Upon Conversion of All Convertible Securities			26,051,238			26,051,238
Shareholders Equity Per Share Upon Conversion Of All Convertible Units (in usd per share)			$ 23.99			$ 23.99
Common Shares Issued And Outstanding Upon Conversion Of All LTIP Units				20,767,565	16,838,151
Shareholders' equity per share upon conversion of LTIPs (in usd per share)				$ 24.38	$ 22.03
Dividends (in usd per share)			$ 3.83	$ 2.50	$ 2.51
Number of Dividends Authorized			4	4	4
Payments of Dividends			(92,067,000)	(47,412,000)	(42,382,000)
Aggregate cost of shares repurchase		10,000,000
Shares repurchased			0	(156,639)	(60,980)	(217,619)
Aggregate cost of shares repurchased			$ 0	$ 3,430,000	$ 1,051,000	$ 4,500,000
Average price per share repurchased (in usd per share)						$ 20.59