Common Shares (Narrative) (Details) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended	6 Months Ended
	Jul. 29, 2016	Nov. 30, 2014	Jun. 30, 2015	Jun. 30, 2016	Dec. 31, 2015
Treasury Stock, Shares Repurchased				10,346,000	10,703,000
Treasury Stock, Value of Shares Repurchased				$ 373,914	$ 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				87,400,000
Preferred Stock, Shares Outstanding				0
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