Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Income Statement [Abstract]
Operating revenues	$ 136,238	$ 99,183	$ 433,561	$ 354,712
Operating expenses:
Cost of gas	53,005	20,871	196,616	129,282
Operations and maintenance	38,546	35,962	78,966	74,901
Environmental remediation	2,611	1,893	9,565	6,922
General taxes	7,564	7,438	16,589	16,122
Depreciation and amortization	21,355	20,413	42,440	40,807
Total operating expenses	123,081	86,577	344,176	268,034
Income from operations	13,157	12,606	89,385	86,678
Other income (expense), net	958	513	1,839	(1,796)
Interest expense, net	9,717	9,718	19,593	19,454
Income before income taxes	4,398	3,401	71,631	65,428
Income tax expense	1,669	1,382	28,592	26,768
Net income	2,729	2,019	43,039	38,660
Other comprehensive income:
Amortization of non-qualified employee benefit plan liability, net of taxes of $88 and $126 for the three months ended and $177 and $253 for the six months ended June 30, 2017 and 2016, respectively	137	143	273	337
Comprehensive income	$ 2,866	$ 2,162	$ 43,312	$ 38,997
Average common shares outstanding:
Basic (in shares)	28,648	27,510	28,641	27,479
Diluted (in shares)	28,717	27,632	28,722	27,591
Earnings per share of common stock:
Basic (in dollars per share)	$ 0.10	$ 0.07	$ 1.50	$ 1.41
Diluted (in dollars per share)	0.10	0.07	1.50	1.40
Dividends declared per share of common stock (in dollars per share)	$ 0.47	$ 0.4675	$ 0.94	$ 0.935