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Stockholder's Equity - Accumulated Other Comprehensive Income (Loss) - Reclassifications of Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Thousands | 3 Months Ended
--- | ---
Mar. 31, 2018 | Mar. 31, 2017
Amortization of pension and postretirement items: |  |  |
Tax benefit |  | $ (9,955) | $ (12,516)
Reclassification out of Accumulated Other Comprehensive Income [Member] |  |  |
Amortization of pension and postretirement items: |  |  |
Net of tax |  | (538) | 849
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Defined Benefit Plans Adjustment [Member] |  |  |
Amortization of pension and postretirement items: |  |  |
Prior service cost | [1] | 19 | 19
Net loss | [1] | (149) | (170)
Total before tax |  | (130) | (151)
Tax benefit |  | 36 | 60
Net of tax |  | (94) | (91)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated (Gain) Loss from Commodity Contracts Cash Flows Hedges [Member] |  |  |
Amortization of pension and postretirement items: |  |  |
Total before tax |  | (616) | 1,538
Tax benefit |  | 172 | (598)
Net of tax |  | (444) | 940
Propane Swap Agreement [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated (Gain) Loss from Commodity Contracts Cash Flows Hedges [Member] |  |  |
Amortization of pension and postretirement items: |  |  |
Other Comprehensive Income Loss Adjustments AOCI Swap Agreements | [2] | (464) | 388
Natural Gas Swaps [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated (Gain) Loss from Commodity Contracts Cash Flows Hedges [Member] |  |  |
Amortization of pension and postretirement items: |  |  |
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax | [2] | (450) |
Natural Gas Futures [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated (Gain) Loss from Commodity Contracts Cash Flows Hedges [Member] |  |  |
Amortization of pension and postretirement items: |  |  |
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax | [2] | $ 298 | $ 1,150
[1] | These amounts are included in the computation of net periodic costs (benefits). See Note 9, Employee Benefit Plans, for additional details.
[2] | These amounts are included in the effects of gains and losses from derivative instruments. See Note 12, Derivative Instruments, for additional details.

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