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Retirement Plans (Retirement Income Plan and Retirement Life Plan) (Details) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2012
Mar. 31, 2011
Retirement Income Plan [Member]
    
Defined Benefit Plan Disclosure [Line Items]    
Service cost $ 2,154 $ 2,173
Interest cost 3,230 3,155
Expected return on plan assets (3,547) (3,482)
Amortization of unrecognized prior service cost 38 37
Amortization of unrecognized net loss 1,383 1,101
Net periodic cost 3,258 2,984
Retirement Life Plan [Member]
    
Defined Benefit Plan Disclosure [Line Items]    
Service cost 0 0
Interest cost 74 77
Expected return on plan assets 0 0
Amortization of unrecognized prior service cost 0 0
Amortization of unrecognized net loss 7 4
Net periodic cost $ 81 $ 81