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Stockholders' Equity Accumulated Other Comprehensive Income (loss) - Reclassifications of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Amortization of defined benefit pension and postretirement plan items: |  |  |  |
Income tax benefit |  | $ (20,994) | $ (14,309) | $ (28,341)
Reclassification out of Accumulated Other Comprehensive Income [Member] |  |  |  |
Amortization of defined benefit pension and postretirement plan items: |  |  |  |
Net of tax |  | (2,198) | (166) | (454)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Defined Benefit Plans Adjustment [Member] |  |  |  |
Amortization of defined benefit pension and postretirement plan items: |  |  |  |
Prior service cost | [1] | 77 | 77 | 77
Net gain | [1] | (579) | (636) | (871)
Total before income taxes |  | (502) | (559) | (794)
Income tax benefit |  | 63 | 223 | 320
Net of tax |  | (439) | (336) | (474)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated (Gain) Loss from Commodity Contracts Cash Flows Hedges [Member] |  |  |  |
Amortization of defined benefit pension and postretirement plan items: |  |  |  |
Total before income taxes |  | (2,460) | 329 | 23
Income tax benefit |  | 701 | (159) | (3)
Net of tax |  | (1,759) | 170 | 20
Propane Swap Agreement [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated (Gain) Loss from Commodity Contracts Cash Flows Hedges [Member] |  |  |  |
Amortization of defined benefit pension and postretirement plan items: |  |  |  |
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax | [2] | (647) | 1,607 | (322)
Natural Gas Futures [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated (Gain) Loss from Commodity Contracts Cash Flows Hedges [Member] |  |  |  |
Amortization of defined benefit pension and postretirement plan items: |  |  |  |
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax | [2] | (2,010) | (456) | 345
Natural Gas Swaps [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated (Gain) Loss from Commodity Contracts Cash Flows Hedges [Member] |  |  |  |
Amortization of defined benefit pension and postretirement plan items: |  |  |  |
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax | [2] | $ 197 | $ (822) | $ 0
[1] | These amounts are included in the computation of net periodic benefits. See Note 17, Employee Benefit Plans, for additional details.
[2] | These amounts are included in the effects of gains and losses from derivative instruments. See Note 8, Derivative Instruments, for additional details.

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