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Stockholders' Equity (Details 2) (USD $)
Mar. 31, 2015
Dec. 31, 2014
Accumulated other comprehensive loss    
Net actuarial loss and prior service cost not yet recognized in net periodic benefit cost, net of income tax benefit $ 4,688,000us-gaap_AccumulatedOtherComprehensiveIncomeLossDefinedBenefitPensionAndOtherPostretirementPlansNetOfTax $ 4,711,000us-gaap_AccumulatedOtherComprehensiveIncomeLossDefinedBenefitPensionAndOtherPostretirementPlansNetOfTax
Accumulated unrealized gain on available-for-sale securities, net of income tax expense of $24,000 and $20,000, respectively 35,000us-gaap_AccumulatedOtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax 31,000us-gaap_AccumulatedOtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax
Accumulated other comprehensive loss (4,653,000)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax (4,680,000)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
Income tax benefit of net actuarial loss and prior service cost not yet recognized in net periodic benefit cost 3,122,000dde_AccumulatedOtherComprehensiveIncomeLossDefinedBenefitPensionAndOtherPostretirementPlansTax 3,137,000dde_AccumulatedOtherComprehensiveIncomeLossDefinedBenefitPensionAndOtherPostretirementPlansTax
Income tax expense of accumulated unrealized gain on available-for-sale securities $ 24,000dde_AccumulatedOtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentTax $ 22,000dde_AccumulatedOtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentTax