Lines Of Credit (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended			3 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Two Thousand Twelve Credit Facility [Member]	Dec. 31, 2012 Two Thousand Twelve Credit Facility [Member]	Dec. 31, 2011 Two Thousand Seven Credit Facility [Member]	Dec. 31, 2010 Two Thousand Seven Credit Facility [Member]	Dec. 31, 2012 Canadian Subsidiary Credit Facility [Member]
Line of Credit Facility [Line Items]
Credit Agreement, years					5
Credit facility				$ 300,000	$ 300,000	$ 300,000		$ 2,000
Credit facility, expiration date				Feb. 02, 2017
Credit facility interest accrued, percentage above LIBOR				1.25%	1.25%	0.45%
Proceeds from Lines of Credit					0
Credit facility, commitment fee per annum on daily unused portion, percent				0.15%		0.09%
Aggregate amount that may increase under credit facility				100,000
Line of Credit Facility, Amount Outstanding				0	0	0		0
Line of Credit Facility, Decrease, Repayments	0	95,000	138,000			95,000	138,000
Line of Credit Facility, Interest Rate During Period						0.68%	0.77%
Interest Expense	$ 504	$ 585	$ 1,478		$ 471	$ 585	$ 1,478