Shareholders Equity (Narrative) (Details) - USD ($)		3 Months Ended	9 Months Ended
	Mar. 07, 2016	Sep. 30, 2016	Sep. 30, 2016	Sep. 30, 2015
Class of Stock [Line Items]
Proceeds from Issuance of Common Stock			$ 300,040,000	$ 101,140,000
At The Market Equity Program
Class of Stock [Line Items]
Aggregate Offering Price Of Common Share		$ 300,000,000	$ 300,000,000
Common shares issued		346,555	950,171
Weighted Average Price Per Common Share		$ 159.38	$ 155.48
Proceeds from Issuance of Common Stock		$ 54,600,000	$ 146,100,000
Remaining Capacity To Issue		42,400,000	42,400,000
At The Market Equity Program | Commissions
Class of Stock [Line Items]
Payments of Stock Issuance Costs (less than for additional offering expenses for the three months ending June 30, 2016)		600,000	1,500,000
At The Market Equity Program | Other Offering Costs
Class of Stock [Line Items]
Payments of Stock Issuance Costs (less than for additional offering expenses for the three months ending June 30, 2016)		$ 100,000	$ 100,000
Public Equity Offering
Class of Stock [Line Items]
Common shares issued	1,000,000
Proceeds from Issuance of Common Stock	$ 149,300,000
Price per share	$ 149.43