Condensed Financial Information of Registrant Other Comprehensive Income (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Condensed Statement of Income Captions [Line Items]
Net income (loss)		$ 35,439	$ (83,891)	$ 28,169	$ 61,092	$ 13,886	$ (5,822)	$ 16,178	$ 15,185	$ 40,809	$ 39,427	$ 72,621
Other comprehensive income (loss):
Other post-retirement benefits income (loss), net of tax										(2,091)	529	(52)
Total other comprehensive income (loss), net of tax										(2,091)	529	(52)
Comprehensive income										38,718	39,956	72,569
Parent Company
Condensed Statement of Income Captions [Line Items]
Net income (loss)										40,809	39,427	72,621
Other comprehensive income (loss):
Other post-retirement benefits income (loss), net of tax	[1]									(2,091)	529	(52)
Total other comprehensive income (loss), net of tax	[1]									(2,091)	529	(52)
Comprehensive income	[1]									$ 38,718	$ 39,956	$ 72,569

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