Condensed Financial Information of Registrant - Statements of Comprehensive Income (Loss) (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
Condensed Statement of Income Captions [Line Items]
Net income (loss)		$ (131,918)	$ (14,271)	$ (40,560)	$ (222,337)	$ 35,439	$ (83,891)	$ 28,169	$ 61,092	$ (409,086)	$ 40,809	$ 39,427
Other comprehensive income (loss):
Other post-retirement benefits income (loss), net of tax										(4,324)	(2,091)	529
Total other comprehensive income (loss), net of tax										(4,324)	(2,091)	529
Comprehensive income										(413,410)	38,718	39,956
Parent Company
Condensed Statement of Income Captions [Line Items]
Net income (loss)										(409,086)	40,809	39,427
Other comprehensive income (loss):
Other post-retirement benefits income (loss), net of tax	[1]									(4,324)	(2,091)	529
Total other comprehensive income (loss), net of tax	[1]									(4,324)	(2,091)	529
Comprehensive income	[1]									$ (413,410)	$ 38,718	$ 39,956

[1] Other comprehensive income (loss) relates to benefit plans at our subsidiaries.