Common Shares (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Nov. 30, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Feb. 13, 2017	Sep. 23, 2016
Treasury Stock, Shares		10,330,000	10,703,000
Treasury Stock, Value		$ 373,339	$ 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
Common shares issued		0	0	898
Special Dividend, Per Share						$ 4.25
2014 ATM program [Member]
Maximum aggregate offering price of common shares	$ 331,300
Common shares issued				209,700
Subsequent Event [Member] | 2014 ATM program [Member]
Maximum aggregate offering price of remaining common shares available for sale					$ 315,300
April 2007 Repurchase Plan [Member]
Stock Repurchase Program, Authorized Amount		$ 500,000
Treasury Stock, Shares		4,300,000
Treasury Stock, Value		$ 230,200
April 2007 Repurchase Plan [Member] | Subsequent Event [Member]
Stock Repurchase Program, Remaining Authorized Repurchase Amount					$ 269,800