CONSOLIDATED STATEMENTS OF COMPREHENSIVE (LOSS) INCOME (Unaudited) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net (loss) income	$ (27,581)	$ 48,539	$ 89,023	$ 17,941
Postretirement benefits:
Amortization of prior service cost	92	91	276	276
Amortization of actuarial gain	(12)		(37)
Income tax expense	(20)	(22)	(59)	(68)
Total comprehensive (loss) income	$ (27,521)	$ 48,608	$ 89,203	$ 18,149

Amount after tax of increase (decrease) in equity from transactions and other events and circumstances from net income and other comprehensive income, attributable to parent entity. Excludes changes in equity resulting from investments by owners and distributions to owners.

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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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