Condensed Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Net income	$ 123,192	$ 68,351	$ 226,483	$ 157,179
Derivative instruments
Comprehensive income (loss) before tax	64	(19)	(5,672)	257
Income tax benefit (provision)	0	0	0	0
Other comprehensive income (loss), derivative instruments, net of tax	64	(19)	(5,672)	257
Pension, postretirement and other post-employment benefits
Comprehensive income (loss) before tax	2,736	(8,521)	6,389	(5,367)
Income tax benefit (provision)	0	0	0	0
Other comprehensive income (loss), pension, postretirement and other post-employment benefits, net of tax	2,736	(8,521)	6,389	(5,367)
Available-for-sale securities
Comprehensive income (loss) before tax	102	0	(379)	(387)
Income tax benefit (provision)	0	0	0	0
Other comprehensive income (loss), available-for-sale securities, net of tax	102	0	(379)	(387)
Total other comprehensive income (loss)	2,902	(8,540)	338	(5,497)
Total comprehensive income	$ 126,094	$ 59,811	$ 226,821	$ 151,682