Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Statement [Abstract]
Operating revenues	$ 124,567	$ 134,476	$ 388,202	$ 430,200
Operating expenses:
Cost of gas	42,053	53,005	150,159	196,616
Operations and maintenance	38,028	34,997	77,551	72,443
Environmental remediation	1,882	2,611	6,506	9,565
General taxes	7,729	7,204	17,203	15,883
Revenue taxes	4,780	0	17,209	0
Depreciation and amortization	21,147	20,224	42,022	40,177
Other operating expenses	679	0	1,532	0
Total operating expenses	116,298	118,041	312,182	334,684
Income from operations	8,269	16,435	76,020	95,516
Other income (expense), net	7	(340)	(827)	(763)
Interest expense, net	8,771	9,473	18,045	19,103
Income (loss) before income taxes	(495)	6,622	57,148	75,650
Income tax (benefit) expense	(156)	2,547	15,476	30,178
Net income (loss) from continuing operations	(339)	4,075	41,672	45,472
Loss from discontinued operations, net of tax,	(659)	(1,346)	(1,133)	(2,433)
Net income (loss)	(998)	2,729	40,539	43,039
Other comprehensive income:
Amortization of non-qualified employee benefit plan liability, net of taxes of $56 and $88 for the three months ended and $111 and $177 for the six months ended June 30, 2018 and 2017, respectively	153	137	307	273
Comprehensive income (loss)	$ (845)	$ 2,866	$ 40,846	$ 43,312
Average common shares outstanding:
Basic (in shares)	28,791	28,648	28,772	28,641
Diluted (in shares)	28,791	28,717	28,825	28,722
Earnings (loss) from continuing operations per share of common stock:
Basic (in dollars per share)	$ (0.01)	$ 0.14	$ 1.45	$ 1.58
Diluted (in dollars per share)	(0.01)	0.14	1.45	1.58
Loss from discontinued operations per share of common stock:
Income (Loss) Per Basic Share from Discontinued Operation, Net of Tax	(0.02)	(0.04)	(0.04)	(0.08)
Income (Loss) Per Diluted Share from Discontinued Operation, Net of Tax	(0.02)	(0.04)	(0.04)	(0.08)
Earnings (loss) per share of common stock:
Basic (in dollars per share)	(0.03)	0.10	1.41	1.50
Diluted (in dollars per share)	(0.03)	0.10	1.41	1.50
Dividends declared per share of common stock (in dollars per share)	$ 0.4725	$ 0.4700	$ 0.9450	$ 0.9400