Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating revenues [Abstract]
Operating revenues	$ 762,173	$ 675,967	$ 723,791
Operating expenses: [Abstract]
Cost of gas	324,795	260,588	327,305
Operations and maintenance	165,246	149,974	157,521
Environmental remediation	15,291	13,298	3,513
General taxes	32,012	30,538	30,281
Depreciation and amortization	85,578	82,289	80,923
Impairment of Long-Lived Assets	192,478	0	0
Total operating expenses	815,400	536,687	599,543
Income (loss) from operations	(53,227)	139,280	124,248
Other income (expense), net	5,348	(543)	7,747
Interest expense, net	38,501	39,128	42,539
Income (loss) before income taxes	(86,380)	99,609	89,456
Income tax expense (benefit)	(30,757)	40,714	35,753
Net income (loss)	(55,623)	58,895	53,703
Other comprehensive income: [Abstract]
Change in employee benefit plan liability, net of taxes of $735 for 2017, $452 for 2016, and ($988) for 2015	(2,059)	(744)	1,561
Amortization of non-qualified employee benefit plan liability, net of taxes of $374 for 2017, ($624) for 2016, and ($883) for 2015	572	955	1,353
Comprehensive income (loss)	$ (57,110)	$ 59,106	$ 56,617
Average common shares outstanding: [Abstract]
Basic	28,669	27,647	27,347
Diluted	28,669	27,779	27,417
Earnings per share of common stock: [Abstract]
Basic	$ (1.94)	$ 2.13	$ 1.96
Diluted	(1.94)	2.12	1.96
Dividends declared per share of common stock	$ 1.88	$ 1.87	$ 1.8625