Common Share Capitalization (Narrative) (Details) (USD $)	0 Months Ended			12 Months Ended			41 Months Ended	0 Months Ended
	Sep. 08, 2014	May 14, 2013	Aug. 04, 2011	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2014	Aug. 20, 2012	Sep. 12, 2012
Common Stock Capitalization [Line Items]
Dividends (in usd per share)				$ 3.08	$ 3.83	$ 2.50
Payments of Dividends				$ (86,449,000)	$ (92,067,000)	$ (47,412,000)
Stock Issued During Period, Shares, New Issues	8,000,000	5,000,000.0		8,000,000	5,000,000	4,025,000
Net proceeds from share issuance	188,200,000	125,300,000
Overallotment Option Percentage By Underwriters									15.00%
Aggregate cost of shares repurchase			10,000,000
Shares repurchased				0	0	(156,639)	(217,619)
Aggregate cost of shares repurchased				0	0	3,430,000	4,500,000
Average price per share repurchased (in usd per share)							$ 20.59
Common Shares Issued And Outstanding Upon Conversion of All Convertible Securities				34,090,992	26,051,238		34,090,992
Common Shares Issued And Outstanding Upon Conversion Of All LTIP Units						20,767,565
Secondary public offering [Member]
Common Stock Capitalization [Line Items]
Stock Issued During Period, Shares, New Issues								3,500,000.0
Net proceeds from share issuance								76,700,000
Secondary public offering overallotment option [Member]
Common Stock Capitalization [Line Items]
Stock Issued During Period, Shares, New Issues									525,000
Net proceeds from share issuance									$ 11,100,000