Common Shares (Narrative) (Details) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended	9 Months Ended
	Oct. 30, 2015	Nov. 30, 2014	Sep. 30, 2015	Sep. 30, 2015	Dec. 31, 2014
Treasury Stock, Shares Repurchased			10,712,000	10,712,000	10,975,000
Treasury Stock, Value of Shares Repurchased			$ 387,114	$ 387,114	$ 396,626
Number of common and preferred stock authorized to issue			185,000,000	185,000,000
Common shares, authorized			175,000,000	175,000,000	175,000,000
Preferred shares, authorized			10,000,000	10,000,000
Common Stock, Shares, Outstanding			86,900,000	86,900,000
Preferred Stock, Shares Outstanding			0	0
2014 ATM program [Member]
Maximum aggregate offering price of common shares		$ 331,300
Stock Issued During Period, Shares, New Issues			0	0.0
2014 ATM program [Member] | Subsequent Event [Member]
Stock Issued During Period, Shares, New Issues	0
Maximum aggregate offering price of remaining common shares available for sale	$ 315,300
April 2007 Repurchase Plan [Member]
Treasury stock allowed for repurchase			$ 500,000	$ 500,000
Treasury Stock, Shares Repurchased			4,300,000	4,300,000
Treasury Stock, Value of Shares Repurchased			$ 230,200	$ 230,200
Share Repurchase Program, Remaining Authorized Repurchase Amount			$ 269,800	$ 269,800