STATEMENTS OF INCOME (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Gross Margin
Revenues	$ 1,818,906us-gaap_RegulatedOperatingRevenueGas	$ 1,689,952us-gaap_RegulatedOperatingRevenueGas	$ 1,376,649us-gaap_RegulatedOperatingRevenueGas
Cost of natural gas	991,949	876,944	620,260
Net margin	826,957us-gaap_GrossProfit	813,008us-gaap_GrossProfit	756,389us-gaap_GrossProfit
Operating expenses
Operations and maintenance	420,686	393,072	363,120
Depreciation and amortization	125,722	144,758	130,150
General taxes	55,255us-gaap_UtilitiesOperatingExpenseTaxes	54,830us-gaap_UtilitiesOperatingExpenseTaxes	47,405us-gaap_UtilitiesOperatingExpenseTaxes
Total operating expenses	601,663us-gaap_UtilitiesOperatingExpense	592,660us-gaap_UtilitiesOperatingExpense	540,675us-gaap_UtilitiesOperatingExpense
Operating income	225,294us-gaap_OperatingIncomeLoss	220,348us-gaap_OperatingIncomeLoss	215,714us-gaap_OperatingIncomeLoss
Other income	1,625us-gaap_OtherNonoperatingIncome	6,165us-gaap_OtherNonoperatingIncome	3,664us-gaap_OtherNonoperatingIncome
Other expense	(2,949)us-gaap_OtherNonoperatingExpense	(3,680)us-gaap_OtherNonoperatingExpense	(2,225)us-gaap_OtherNonoperatingExpense
Interest expense	(45,842)us-gaap_InterestExpense	(61,366)us-gaap_InterestExpense	(60,793)us-gaap_InterestExpense
Income before income taxes	178,128	161,467	156,360
Income taxes	(68,338)us-gaap_IncomeTaxExpenseBenefit	(62,272)us-gaap_IncomeTaxExpenseBenefit	(59,851)us-gaap_IncomeTaxExpenseBenefit
Net income	$ 109,790us-gaap_NetIncomeLoss	$ 99,195us-gaap_NetIncomeLoss	$ 96,509us-gaap_NetIncomeLoss
Earnings per share (Note 7)
Basic	$ 2.10us-gaap_EarningsPerShareBasic	$ 1.90us-gaap_EarningsPerShareBasic	$ 1.84us-gaap_EarningsPerShareBasic
Diluted	$ 2.07us-gaap_EarningsPerShareDiluted	$ 1.90us-gaap_EarningsPerShareDiluted	$ 1.84us-gaap_EarningsPerShareDiluted
Average shares (thousands)
Basic	52,364	52,319	52,319
Diluted	52,946	52,319	52,319
Dividends declared per share of stock	$ 0.84	$ 0.00	$ 0.00

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