Common Shares (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	May 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Feb. 15, 2019	Sep. 23, 2016	Aug. 31, 2015
Treasury Stock, Shares, Acquired		3,222
Treasury Stock, Value, Acquired, Par Value Method		$ 253
Treasury Stock, Shares		9,841,000	10,073,000
Treasury Stock, Value		$ 355,804	$ 364,066
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		93,200,000
Preferred Stock, Shares Outstanding		0
Common shares issued		0	4,778	0
Proceeds from issuance of common shares		$ 0	$ 445,038	$ 0
Special Dividend, Per Share						$ 4.25
2017 ATM program [Member]
Maximum aggregate offering price of common shares	$ 315,300
2014 ATM program [Member]
Common shares issued		28,100
Subsequent Event [Member] | 2017 ATM program [Member]
Maximum aggregate offering price of remaining common shares available for sale					$ 312,800
April 2007 Repurchase Plan [Member]
Stock Repurchase Program, Authorized Amount		$ 500,000
April 2007 Repurchase Plan [Member] | Subsequent Event [Member]
Stock Repurchase Program, Remaining Authorized Repurchase Amount					$ 269,500
Unsecured Credit Facility [Member]
Line of Credit Facility, Current Borrowing Capacity		$ 600,000					$ 500,000