Condensed Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Net income (loss)	$ (344,615)	$ 233,799	$ 312,577
Derivative instruments
Comprehensive income (loss) before tax	(1,328)	(5,892)	2,681
Income tax benefit (provision)	0	0	0
Other comprehensive income (loss), derivative instruments, net of tax	(1,328)	(5,892)	2,681
Pension, postretirement and other post-employment benefits
Comprehensive income (loss) before tax	(39,732)	(32,038)	20,591
Income tax benefit (provision)	0	0	0
Other comprehensive income (loss), pension, postretirement and other post-employment benefits, net of tax	(39,732)	(32,038)	20,591
Available-for-sale securities
Comprehensive income (loss) before tax	(330)	323	(343)
Income tax benefit (provision)	0	0	0
Other comprehensive income (loss), available-for-sale securities, net of tax	(330)	323	(343)
Total other comprehensive income (loss)	(41,390)	(37,607)	22,929
Total comprehensive income (loss)	$ (386,005)	$ 196,192	$ 335,506