Common Shares (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended		3 Months Ended	9 Months Ended
	Oct. 28, 2016	Nov. 30, 2014	Sep. 30, 2015	Sep. 30, 2016	Sep. 23, 2016	Dec. 31, 2015
Treasury Stock, Shares Repurchased				10,337,000		10,703,000
Treasury Stock, Value of Shares Repurchased				$ 373,597		$ 386,793
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				87,500,000
Preferred Stock, Shares Outstanding				0
Dividend Payable, Amount Per Share				$ 0.75
Special Dividend, Per Share					$ 4.25
2014 ATM program [Member]
Maximum aggregate offering price of common shares		$ 331,300
Common shares issued			0	0
2014 ATM program [Member] | Subsequent Event [Member]
Maximum aggregate offering price of remaining common shares available for sale	$ 315,300
Common shares issued	0
April 2007 Repurchase Plan [Member]
Treasury stock allowed for repurchase				$ 500,000
Treasury Stock, Shares Repurchased				4,300,000
Treasury Stock, Value of Shares Repurchased				$ 230,200
April 2007 Repurchase Plan [Member] | Subsequent Event [Member]
Share Repurchase Program, Remaining Authorized Repurchase Amount	$ 269,800