Common Shares (Narrative) (Details) - USD ($) $ in Thousands	1 Months Ended			3 Months Ended
	May 04, 2018	May 31, 2017	Nov. 30, 2014	Mar. 31, 2018	Dec. 31, 2017	May 05, 2017
Common shares repurchased				$ 253
Treasury Stock, Shares, Acquired				3,222
Number of common and preferred stock authorized to issue				185,000,000
Common shares, authorized				175,000,000	175,000,000
Preferred shares, authorized				10,000,000
Common Stock, Shares, Outstanding				92,800,000
Preferred Stock, Shares Outstanding				0
Unsecured Credit Facility [Member]
Line of Credit Facility, Current Borrowing Capacity				$ 600,000
2014 ATM program [Member]
Maximum aggregate offering price of remaining common shares available for sale						$ 315,300
Maximum aggregate offering price of common shares			$ 331,300
2017 ATM program [Member]
Maximum aggregate offering price of common shares		$ 315,300
2017 ATM program [Member] | Subsequent Event [Member]
Common shares issued	0
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