Common Shares (Narrative) (Details) - USD ($) $ in Thousands	1 Months Ended		2 Months Ended	12 Months Ended
	Nov. 30, 2014	May. 31, 2012	Feb. 19, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Treasury Stock, Shares				10,703,000	10,975,000
Treasury Stock, Value				$ 386,793	$ 396,626
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				86,900,000
Common shares issued				0	898	555
2014 ATM program [Member]
Maximum aggregate offering price of common shares	$ 331,300
Common shares issued				0	209,700
2012 ATM Program [Member]
Maximum aggregate offering price of common shares		$ 300,000
Common shares issued					688,300.0	555,100
Subsequent Event [Member] | 2014 ATM program [Member]
Maximum aggregate offering price of remaining common shares available for sale			$ 315,300
Common shares issued			0
April 2007 Repurchase Plan [Member]
Stock Repurchase Program, Authorized Amount				$ 500,000
Treasury Stock, Shares				4,300,000
Treasury Stock, Value				$ 230,200
Stock Repurchase Program, Remaining Authorized Repurchase Amount				$ 269,800