Condensed Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Net income	$ 72,741	$ 59,985
Derivative instruments
Comprehensive income (loss) before tax	2,717	6,557
Income tax benefit (provision)	0	0
Other comprehensive income (loss), derivative instruments, net of tax	2,717	6,557
Pension, postretirement and other post-employment benefits
Comprehensive income (loss) before tax	0	0
Income tax benefit (provision)	0	0
Other comprehensive income (loss), pension, postretirement and other post-employment benefits, net of tax	0	0
Available-for-sale securities
Comprehensive income (loss) before tax	377	(658)
Income tax benefit (provision)	0	0
Other comprehensive income (loss), available-for-sale securities, net of tax	377	(658)
Total other comprehensive income (loss)	3,094	5,899
Total comprehensive income	$ 75,835	$ 65,884