Shareholders' Equity Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	May 07, 2018
Class of Stock [Line Items]
Net cash proceeds of common stock		$ 81,628	$ 51,189
At The Market Equity Program
Class of Stock [Line Items]
Aggregate offering price of common share				$ 400,000
Shares sold	413,395	612,658
Weighted average price per common share	$ 129.010	$ 127.86
Net cash proceeds of common stock	$ 52,800	$ 77,400
Remaining capacity to issue	321,700	321,700
At The Market Equity Program | Commissions
Class of Stock [Line Items]
Commissions related to sales of common shares	500	800
At The Market Equity Program | Other offering costs
Class of Stock [Line Items]
Commissions related to sales of common shares	$ 100	$ 200