Condensed Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Net income	$ 59,985	$ 51,668
Derivative instruments
Comprehensive income (loss) before tax	6,557	19
Income tax benefit (provision)	0	0
Derivatives qualifying as hedges, adjustment, net of tax	6,557	19
Pension, postretirement and other post-employment benefits
Comprehensive income (loss) before tax	0	0
Income tax benefit (provision)	0	0
Pension and other postretirement benefit plans, adjustment, net of tax	0	0
Available-for-sale securities
Comprehensive income (loss) before tax	(658)	(387)
Income tax benefit (provision)	0	0
Available-for-sale securities adjustment, net of tax	(658)	(387)
Total other comprehensive income (loss)	5,899	(368)
Total comprehensive income (loss)	$ 65,884	$ 51,300