Common Shares (Narrative) (Details) - USD ($) shares in Thousands, $ in Thousands	1 Months Ended	9 Months Ended
	May 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Nov. 01, 2019	Dec. 31, 2018
Number of common and preferred stock authorized to issue		185,000
Common shares, authorized		175,000			175,000
Preferred shares, authorized		10,000
Common Stock, Shares, Outstanding		96,800
Preferred Stock, Shares Outstanding		0
Stock Issued During Period, Shares, New Issues		3,400
Proceeds from issuance of common shares		$ 328,374	$ 0
Unsecured Credit Facility [Member]
Maximum borrowing capacity under unsecured credit facility		900,000
2017 ATM program [Member]
Maximum aggregate offering price of common shares	$ 315,300
2017 ATM program [Member] | Subsequent Event [Member]
Maximum aggregate offering price of remaining common shares available for sale				$ 312,800
April 2007 Repurchase Plan [Member]
Treasury stock allowed for repurchase		$ 500,000
April 2007 Repurchase Plan [Member] | Subsequent Event [Member]
Share Repurchase Program, Remaining Authorized Repurchase Amount				$ 269,500