Condended Financial Information of Registrant Other Comprehensive Income (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Statement of Income Captions [Line Items]
Net income (loss)		$ 13,886	$ (5,822)	$ 16,178	$ 15,185	$ 19,005	$ 18,824	$ 7,006	$ 27,786	$ 39,427	$ 72,621	$ (45,835)
Other comprehensive income (loss):
Other post-retirement benefits income (loss), net of tax										529	(52)	2,196
Total other comprehensive income (loss), net of tax										529	(52)	2,196
Comprehensive income (loss)										39,956	72,569	(43,639)
Parent Company
Condensed Statement of Income Captions [Line Items]
Net income (loss)										39,427	72,621	(45,835)
Other comprehensive income (loss):
Other post-retirement benefits income (loss), net of tax	[1]									529	(52)	2,196
Total other comprehensive income (loss), net of tax	[1]									529	(52)	2,196
Comprehensive income (loss)	[1]									$ 39,956	$ 72,569	$ (43,639)

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