Shareholders' Equity (Details) - USD ($)	3 Months Ended	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2015	Jun. 30, 2014	May. 11, 2015
Class of Stock
Proceeds from issuance of stock		$ 58,007,000	$ 103,931,000
At The Market Equity Program
Class of Stock
Aggregate offering price (in usd)				$ 300,000,000
Common shares issued (in shares)	427,134	427,134
Weighted average price per share	$ 134.12	$ 134.12
Proceeds from issuance of stock	$ 56,600,000	$ 56,600,000
Remaining capacity to issue (in usd)	242,700,000	242,700,000
At The Market Equity Program | Commissions
Class of Stock
Stock issuance costs	600,000	600,000
At The Market Equity Program | Other Offering Costs
Class of Stock
Stock issuance costs	$ 100,000	$ 100,000