Accumulated Other Comprehensive Income - Schedule of Amounts Reclassified from Accumulated Other Comprehensive Income into Consolidated Statement of Operations (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Line items in income statement impacted by reclassifications out of accumulated other comprehensive income
General and administrative										$ 105,391	$ 94,870	$ 96,641
Amortization of net (gain) loss	[1]									28	0	0
Income before income taxes										241,123	900,224	698,137
Income tax expense										(40,594)	(219,154)	$ (141,094)
Accounting Standards Update [Extensible List]												us-gaap:AccountingStandardsUpdate201802Member
Net income		$ 131,206	$ (14,961)	$ 30,374	$ 53,910	$ 146,940	$ 90,358	$ 181,009	$ 262,763	200,529	681,070	$ 557,043
Reclassification out of Accumulated Other Comprehensive Income
Line items in income statement impacted by reclassifications out of accumulated other comprehensive income
Amortization of net (gain) loss										36	0	0
Amortization of prior service cost | Reclassification out of Accumulated Other Comprehensive Income
Line items in income statement impacted by reclassifications out of accumulated other comprehensive income
General and administrative										709	709	709
Postretirement Benefits | Reclassification out of Accumulated Other Comprehensive Income
Line items in income statement impacted by reclassifications out of accumulated other comprehensive income
Income before income taxes										745	709	709
Income tax expense										(170)	(162)	(162)
Cumulative effect of adoption of ASU 2018-02 reclassified to retained earnings										0	0	(446)
Net income										$ 575	$ 547	$ 101

[1] Net of income taxes of $8 for the year ended December 31, 2020.