Capital Stock and Stock Options (Details 4) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Amounts Reclassified from AOCI
Amortization of net loss on terminated derivative instruments designated as cash flow hedges	$ 783	$ 800
Income tax expense	(277)	(282)
Net income	37,520	32,927
[AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember] | [ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember]
Amounts Reclassified from AOCI
Amortization of net loss on terminated derivative instruments designated as cash flow hedges	783
Income tax expense	(277)
Net income	$ 506