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Components of Net Periodic Benefit Cost (Details) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Pension Benefits [Member]
    
Defined Benefit Plan Disclosure [Line Items]    
Service cost $ 559 $ 1,218
Interest cost 846 917
Expected return on plan assets (827) (815)
Amortization of transition obligation 0 0
Net amortization of prior service cost (4) 41
Net amortization of net (gain) loss 523 567
Net Periodic Benefit Cost 1,097 1,928
Post-Retirement Health Benefits [Member]
    
Defined Benefit Plan Disclosure [Line Items]    
Service cost 17 15
Interest cost 43 43
Expected return on plan assets 0 0
Amortization of transition obligation 15 15
Net amortization of prior service cost 0 0
Net amortization of net (gain) loss 0 0
Net Periodic Benefit Cost $ 75 $ 73