Common Shares (Narrative) (Details) - USD ($) shares in Thousands, $ in Thousands	1 Months Ended	3 Months Ended
	May 31, 2017	Mar. 31, 2020	Mar. 31, 2019	May 08, 2020	Dec. 31, 2019
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		97,400
Preferred Stock, Shares Outstanding		0
Proceeds from issuance of common shares		$ 0	$ 328,374
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				$ 287,700
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