Condensed Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Net income	$ 43,306	$ 37,160	$ 103,291	$ 88,828
Derivative instruments
Comprehensive income (loss) before tax	(12,293)	257	(5,736)	276
Income tax benefit (provision)	0	0	0	0
Other comprehensive income (loss), derivative instruments, net of tax	(12,293)	257	(5,736)	276
Pension, postretirement and other post-employment benefits
Comprehensive income (loss) before tax	3,653	3,154	3,653	3,154
Income tax benefit (provision)	0	0	0	0
Other comprehensive income (loss), pension, postretirement and other post-employment benefits, net of tax	3,653	3,154	3,653	3,154
Available-for-sale securities
Comprehensive income (loss) before tax	177	0	(481)	(387)
Income tax benefit (provision)	0	0	0	0
Other comprehensive income (loss), available-for-sale securities, net of tax	177	0	(481)	(387)
Total other comprehensive income (loss)	(8,463)	3,411	(2,564)	3,043
Total comprehensive income	$ 34,843	$ 40,571	$ 100,727	$ 91,871