Condensed Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Net income	$ 106,769	$ 123,192	$ 242,350	$ 226,483
Derivative instruments
Comprehensive income (loss) before tax	(2,360)	64	(2,421)	(5,672)
Income tax benefit (provision)	0	0	0	0
Other comprehensive income (loss), derivative instruments, net of tax	(2,360)	64	(2,421)	(5,672)
Pension, postretirement and other post-employment benefits
Comprehensive income (loss) before tax	(4,019)	2,736	(1,117)	6,389
Income tax benefit (provision)	0	0	0	0
Other comprehensive income (loss), pension, postretirement and other post-employment benefits, net of tax	(4,019)	2,736	(1,117)	6,389
Available-for-sale securities
Comprehensive income (loss) before tax	(98)	102	553	(379)
Income tax benefit (provision)	0	0	0	0
Other comprehensive income (loss), available-for-sale securities, net of tax	(98)	102	553	(379)
Total other comprehensive income (loss)	(6,477)	2,902	(2,985)	338
Total comprehensive income	$ 100,292	$ 126,094	$ 239,365	$ 226,821