Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating revenues [Abstract]
Operating revenues	$ 723,791	$ 754,037	$ 758,518
Operating expenses: [Abstract]
Cost of Gas	327,305	365,490	373,298
Operations and maintenance	157,521	136,982	136,613
Environmental Remediation Expense	3,513	0	0
General taxes	30,281	29,407	29,956
Depreciation and amortization	80,923	79,193	75,905
Operating Costs and Expenses	599,543	611,072	615,772
Income from operations	124,248	142,965	142,746
Other income and expense, net	7,747	1,933	4,669
Interest Income (Expense), Nonoperating, Net	42,539	44,563	45,172
Income (Loss) from Continuing Operations before Income Taxes, Extraordinary Items, Noncontrolling Interest	89,456	100,335	102,243
Income tax expense	(35,753)	(41,643)	(41,705)
Net Income	53,703	58,692	60,538
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	56,617	54,974	63,471
Other comprehensive income: [Abstract]
Change in employee benefit plan liability, net of taxes of ($988) for 2015, $2,857 for 2014, and ($1,304) for 2013	1,561	(4,364)	1,998
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, Net of Tax of ($883) for 2015, ($438) for 2014, and ($608) for 2013	$ 1,353	$ 646	$ 935
Average common shares outstanding: [Abstract]
Basic	27,347	27,164	26,974
Diluted	27,417	27,223	27,027
Earnings per share of common stock: [Abstract]
Basic	$ 1.96	$ 2.16	$ 2.24
Diluted	1.96	2.16	2.24
Dividends declared per share of common stock	$ 1.86	$ 1.85	$ 1.83