Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Statement of Comprehensive Income [Abstract]
Net Income (Loss)	$ (106,904)	$ 57,833	$ (251,276)	$ 94,191
Other comprehensive loss, net:
Net change related to employee benefit plans, net of tax expense	736	669	1,334	1,245
Net change in derivative instruments, net of tax expense/benefit	(2,419)	(3,646)	(2,075)	(2,501)
Net change in available-for-sale investments, net of tax expense/benefit	1,065	769	1,454	1,309
Net current-period other comprehensive income (loss)	(618)	(2,208)	713	53
Total Comprehensive Income (Loss)	$ (107,522)	$ 55,625	$ (250,563)	$ 94,244