CONSOLIDATED STATEMENTS OF COMPREHENSIVE (LOSS) INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement of Comprehensive Income [Abstract]
Net (loss) income	$ (199,897)	$ 149,630	$ 230,155
Postretirement benefit plans:
Net actuarial gain (loss) arising during the year	6,594	(3,805)	7,011
Less reclassification of amounts from other comprehensive income (loss) to net (loss) income:
Amortization of prior service credit	(1,054)	(853)	(1,049)
Amortization of net actuarial loss	2,583	1,825	2,893
Postretirement benefit plans	8,123	(2,833)	8,855
Interest rate swap:
Unrealized loss arising during the year	(1,040)	0	0
Reclassification of realized gains on interest rate swap from other comprehensive income (loss) to net (loss) income	(57)	0	0
Interest rate swap	(1,097)	0	0
Unrealized holding gain (loss) on debt securities arising during the year	48	(1)	(109)
Unrealized foreign currency translation adjustment	1,558	(9,281)	4,028
Other comprehensive income (loss)	8,632	(12,115)	12,774
Comprehensive (loss) income	(191,265)	137,515	242,929
Postretirement benefit plans:
Net actuarial gain (loss) arising during the year	(2,321)	1,339	(2,465)
Less reclassification of amounts from other comprehensive income (loss) to net (loss) income:
Amortization of prior service credit	367	568	372
Amortization of net actuarial loss	(640)	(1,059)	(744)
Postretirement benefit plans	(2,594)	848	(2,837)
Interest rate swap:
Unrealized loss arising during the year	364	0	0
Reclassification of realized gains on interest rate swap from other comprehensive income (loss) to net (loss) income	20	0	0
Interest rate swap	384	0	0
Unrealized holding gain (loss) on debt securities arising during the year	(17)	0	38
Total net tax (expense) benefit included in other comprehensive income (loss)	$ (2,227)	$ 848	$ (2,799)