Accumulated Other Comprehensive Income (Loss) - Reclassifications Out of AOCI (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Line items in income statement impacted by reclassifications out of accumulated other comprehensive income
General and administrative expense	$ (96,641)	$ (97,786)	$ (85,633)
Income (loss) before income taxes	698,137	(228,435)	(659,599)
Income tax benefit (expense)	(141,094)	328,828	242,475
Cumulative impact from accounting change	(446)
NET INCOME (LOSS)	557,043	100,393	(417,124)
Amount Reclassified from Accumulated Other Comprehensive Income
Line items in income statement impacted by reclassifications out of accumulated other comprehensive income
Income (loss) before income taxes	709	2,733	(111)
Income tax benefit (expense)	(162)	(1,010)	41
Cumulative impact from accounting change	(446)	0	0
Postretirement Benefits | Amount Reclassified from Accumulated Other Comprehensive Income
Line items in income statement impacted by reclassifications out of accumulated other comprehensive income
NET INCOME (LOSS)	101	1,723	(70)
Amortization of prior service cost | Amount Reclassified from Accumulated Other Comprehensive Income
Line items in income statement impacted by reclassifications out of accumulated other comprehensive income
General and administrative expense	$ 709	$ 2,733	$ (111)