Consolidated Statements of Comprehensive Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Operating revenues [Abstract]
Operating revenues	$ 297,323	$ 255,529
Operating expenses: [Abstract]
Cost of Gas	143,611	108,411
Operations and maintenance	40,420	38,939
Environmental Remediation Expense	6,954	5,029
General taxes	9,025	8,684
Depreciation and amortization	21,085	20,394
Operating Costs and Expenses	221,095	181,457
Income from operations	76,228	74,072
Other income and (expense), net	881	(2,309)
Interest Income (Expense), Nonoperating, Net	9,876	9,736
Income (Loss) from Continuing Operations before Income Taxes, Extraordinary Items, Noncontrolling Interest	67,233	62,027
Income tax expense	26,923	25,386
Net Income	40,310	36,641
Other comprehensive income: [Abstract]
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, Net of Tax of $89 and $127 for the three months ended March 31, 2017 and 2016, respectively	136	194
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	$ 40,446	$ 36,835
Average common shares outstanding: [Abstract]
Basic	28,633	27,448
Diluted	28,723	27,560
Earnings per share of common stock: [Abstract]
Basic	$ 1.41	$ 1.33
Diluted	1.40	1.33
Dividends declared per share of common stock	$ 0.4700	$ 0.4675