Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Operating revenues [Abstract]
Operating revenues	$ 255,529	$ 261,665
Operating expenses: [Abstract]
Cost of Gas	108,411	125,705
Operations and maintenance	38,939	54,116
Environmental Remediation Expense	5,029	0
General taxes	8,684	8,732
Depreciation and amortization	20,394	20,111
Operating Costs and Expenses	181,457	208,664
Income from operations	74,072	53,001
Other income and expense, net	(2,309)	5,049
Interest Income (Expense), Nonoperating, Net	9,736	10,481
Income (Loss) from Continuing Operations before Income Taxes, Extraordinary Items, Noncontrolling Interest	62,027	47,569
Income tax expense	(25,386)	(19,083)
Net Income	36,641	28,486
Other comprehensive income: [Abstract]
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, Net of Tax of $127 and $216 for the three months ended March 31, 2016 and 2015, respectively	194	332
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	$ 36,835	$ 28,818
Average common shares outstanding: [Abstract]
Basic	27,448	27,301
Diluted	27,560	27,369
Earnings per share of common stock: [Abstract]
Basic	$ 1.33	$ 1.04
Diluted	1.33	1.04
Dividends declared per share of common stock	$ 0.4675	$ 0.4650