Common Shares (Narrative) (Details) - USD ($) shares in Thousands, $ in Thousands		1 Months Ended	6 Months Ended
	Jun. 04, 2020	May 31, 2017	Jun. 30, 2020	Jun. 30, 2019	Jul. 31, 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	$ 362,700
2017 ATM program [Member] | Subsequent Event [Member]
Maximum aggregate offering price of remaining common shares available for sale					$ 362,700
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