Common Shares (Narrative) (Details) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended
	Apr. 30, 2016	Nov. 30, 2014	Mar. 31, 2016	Mar. 31, 2015	Apr. 29, 2016	Dec. 31, 2015
Treasury Stock, Shares Repurchased			10,460,000			10,703,000
Treasury Stock, Value of Shares Repurchased			$ 378,032			$ 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			87,000,000
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]
Common shares issued	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
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