Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 233,799	$ 312,577	$ 238,450
Derivative instruments
Comprehensive income (loss) before tax	(5,892)	2,681	647
Income tax benefit (provision)	0	0	0
Derivative instruments, net of tax	(5,892)	2,681	647
Pension, postretirement and other post-employment benefits
Comprehensive income (loss) before tax	(32,038)	20,591	(4,347)
Income tax benefit (provision)	0	0	0
Pension, postretirement and other post-employment benefits, net of tax	(32,038)	20,591	(4,347)
Available-for-sale securities
Comprehensive income (loss) before tax	323	(343)	(387)
Income tax benefit (provision)	0	0	0
Available-for-sale securities, net of tax	323	(343)	(387)
Total other comprehensive income (loss)	(37,607)	22,929	(4,087)
Total comprehensive income	$ 196,192	$ 335,506	$ 234,363