Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Statement of Comprehensive Income [Abstract]
Net Income (Loss)	$ (97,099)	$ 80,076	$ (348,375)	$ 174,267
Other comprehensive income (loss), net:
Net change related to employee benefit plans, net of tax expense	(11,284)	786	(9,950)	2,031
Net change in derivative instruments, net of tax expense/benefit	(1,266)	2,096	(3,341)	(405)
Net change in available-for-sale investments, net of tax expense/benefit	(579)	92	875	1,401
Net current-period other comprehensive income	(13,129)	2,974	(12,416)	3,027
Total Comprehensive Income (Loss)	$ (110,228)	$ 83,050	$ (360,791)	$ 177,294