Condensed Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Net income	$ 62,840	$ 43,306	$ 135,581	$ 103,291
Derivative instruments
Comprehensive income (loss) before tax	(2,778)	(12,293)	(61)	(5,736)
Income tax benefit (provision)	0	0	0	0
Other comprehensive income (loss), derivative instruments, net of tax	(2,778)	(12,293)	(61)	(5,736)
Pension, postretirement and other post-employment benefits
Comprehensive income (loss) before tax	2,902	3,653	2,902	3,653
Income tax benefit (provision)	0	0	0	0
Other comprehensive income (loss), pension, postretirement and other post-employment benefits, net of tax	2,902	3,653	2,902	3,653
Available-for-sale securities
Comprehensive income (loss) before tax	274	177	651	(481)
Income tax benefit (provision)	0	0	0	0
Other comprehensive income (loss), available-for-sale securities, net of tax	274	177	651	(481)
Total other comprehensive income (loss)	398	(8,463)	3,492	(2,564)
Total comprehensive income	$ 63,238	$ 34,843	$ 139,073	$ 100,727