Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Statement [Abstract]
Operating revenues	$ 88,190	$ 87,727	$ 521,751	$ 442,439
Operating expenses:
Cost of gas	27,239	28,264	223,855	157,546
Operations and maintenance	36,867	34,870	115,833	109,771
Environmental remediation	1,355	1,191	10,920	8,113
General taxes	7,901	7,211	24,490	23,333
Depreciation and amortization	21,484	20,628	63,924	61,435
Total operating expenses	94,846	92,164	439,022	360,198
Income (loss) from operations	(6,656)	(4,437)	82,729	82,241
Other income (expense), net	1,493	652	3,332	(1,144)
Interest expense, net	9,451	9,729	29,044	29,183
Income (loss) before income taxes	(14,614)	(13,514)	57,017	51,914
Income tax expense (benefit)	(6,119)	(5,474)	22,473	21,294
Net income (loss)	(8,495)	(8,040)	34,544	30,620
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Arising During Period, Net of Tax	0	(1,086)	0	(1,086)
Other comprehensive income (loss):
Amortization of non-qualified employee benefit plan liability, net of taxes of $98 and $223 for the three months ended and $275 and $477 for the nine months ended September 30, 2017 and 2016, respectively	150	341	423	678
Comprehensive income (loss)	$ (8,345)	$ (8,785)	$ 34,967	$ 30,212
Average common shares outstanding:
Basic (in shares)	28,678	27,554	28,653	27,504
Diluted (in shares)	28,678	27,554	28,734	27,629
Earnings (loss) per share of common stock:
Basic (in dollars per share)	$ (0.30)	$ (0.29)	$ 1.21	$ 1.11
Diluted (in dollars per share)	(0.30)	(0.29)	1.20	1.11
Dividends declared per share of common stock (in dollars per share)	$ 0.4700	$ 0.4675	$ 1.4100	$ 1.4025