Shareholders' Equity (Details) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2015	Sep. 30, 2014	May. 11, 2015
Class of Stock
Proceeds from issuance of stock		$ 101,140,000	$ 154,250,000
At The Market Equity Program
Class of Stock
Aggregate offering price (in usd)				$ 300,000,000
Common shares issued (in shares)	323,273	750,407
Weighted average price per share	$ 134.35	$ 134.21
Proceeds from issuance of stock	$ 42,900,000	$ 99,600,000
Remaining capacity to issue (in usd)	199,300,000	199,300,000
At The Market Equity Program | Commissions
Class of Stock
Payments of Stock Issuance Costs (less than for additional offering expenses for the three months ended September 30, 2015)	400,000	1,000,000
At The Market Equity Program | Other Offering Costs
Class of Stock
Payments of Stock Issuance Costs (less than for additional offering expenses for the three months ended September 30, 2015)	$ 100,000	$ 100,000