Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating revenues [Abstract]
Operating revenues	$ 675,967	$ 723,791	$ 754,037
Operating expenses: [Abstract]
Cost of Gas	260,588	327,305	365,490
Operations and maintenance	149,974	157,521	136,982
Environmental Remediation Expense	13,298	3,513	0
General taxes	30,538	30,281	29,407
Depreciation and amortization	82,289	80,923	79,193
Operating Costs and Expenses	536,687	599,543	611,072
Income from operations	139,280	124,248	142,965
Other income and (expense), net	(543)	7,747	1,933
Interest Income (Expense), Nonoperating, Net	39,128	42,539	44,563
Income (Loss) from Continuing Operations before Income Taxes, Extraordinary Items, Noncontrolling Interest	99,609	89,456	100,335
Income tax expense	40,714	35,753	41,643
Net Income	58,895	53,703	58,692
Other comprehensive income: [Abstract]
Change in employee benefit plan liability, net of taxes of $452 for 2016, ($988) for 2015, and $2,857 for 2014	(744)	1,561	(4,364)
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, Net of Tax of ($624) for 2016, ($883) for 2015, and ($438) for 2014	955	1,353	646
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	$ 59,106	$ 56,617	$ 54,974
Average common shares outstanding: [Abstract]
Basic	27,647	27,347	27,164
Diluted	27,779	27,417	27,223
Earnings per share of common stock: [Abstract]
Basic	$ 2.13	$ 1.96	$ 2.16
Diluted	2.12	1.96	2.16
Dividends declared per share of common stock	$ 1.87	$ 1.86	$ 1.845