Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Other Comprehensive Income (Loss) (AOCI) by component, net of tax
Beginning balance, January 1			$ 13,807
Postretirement medical plan change	$ 24,659	$ 0	24,659	$ 0
Ending balance, June 30	14,702		14,702
Postretirement medical plan \| Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Negative prior service cost base	24,400		$ 24,400
Amortization period in months			1 year 5 months
Other Comprehensive Income (Loss) (AOCI) by component, net of tax
Beginning balance, January 1			$ 13,807	21,884
Amounts reclassified from accumulated other comprehensive income (loss)			(18,298)	(5,462)
Postretirement medical plan change			24,659	0
Tax expense (benefit)			(5,466)	0
Net current period other comprehensive income (loss)			895	(5,462)
Ending balance, June 30	$ 14,702	$ 16,422	$ 14,702	$ 16,422

No definition available.

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Number of months new negative prior service costs in addition to actuarial loss in AOCI will be amortized

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Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at period end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, unrealized gains and losses on certain investments in debt and equity securities, other than temporary impairment (OTTI) losses related to factors other than credit losses on available-for-sale and held-to-maturity debt securities that an entity does not intend to sell and it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis, as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge.

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Amount, before tax, of accumulated other comprehensive (income) loss for cost (credit) of benefit change attributable to participants' prior service from plan amendment or plan initiation of defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

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Amount, before tax, of cost (credit) of benefit change attributable to participants' prior service from plan amendment or plan initiation of defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

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Amount after tax and reclassification adjustments of other comprehensive income (loss).

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Amount of tax (expense) benefit for cost (credit) of benefit change attributable to participants' prior service from plan amendment or plan initiation of defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

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Amount after tax of reclassification adjustments of other comprehensive income (loss).

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