Common Shares (Narrative) (Details) - USD ($) shares in Thousands, $ in Thousands	1 Months Ended	3 Months Ended
	May 31, 2017	Mar. 31, 2019	Mar. 31, 2018	May 03, 2019	Dec. 31, 2018
Common shares repurchased			$ 253
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		96,700
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