Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Statement [Abstract]
Operating revenues	$ 264,712	$ 297,323
Operating expenses:
Cost of gas	108,106	143,611
Operations and maintenance	40,559	39,116
Environmental remediation	4,624	6,954
General taxes	9,808	9,025
Revenue taxes	12,429	0
Depreciation and amortization	20,985	21,085
Other operating expenses	853	0
Total operating expenses	197,364	219,791
Income from operations	67,348	77,532
Other income (expense), net	(834)	(423)
Interest expense, net	9,515	9,876
Income before income taxes	56,999	67,233
Income tax expense	15,462	26,923
Net income	41,537	40,310
Other comprehensive income:
Amortization of non-qualified employee benefit plan liability, net of taxes of $55 and $89 for the three months ended March 31, 2018 and 2017, respectively	154	136
Comprehensive income	$ 41,691	$ 40,446
Average common shares outstanding:
Basic (in shares)	28,753	28,633
Diluted (in shares)	28,803	28,723
Earnings per share of common stock:
Basic (in dollars per share)	$ 1.44	$ 1.41
Diluted (in dollars per share)	1.44	1.40
Dividends declared per share of common stock (in dollars per share)	$ 0.4725	$ 0.4700