STATEMENTS OF INCOME (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Gross Margin
Revenues	$ 676,531us-gaap_RegulatedOperatingRevenueGas	$ 766,178us-gaap_RegulatedOperatingRevenueGas
Cost of natural gas	413,553	506,342
Net margin	262,978us-gaap_GrossProfit	259,836us-gaap_GrossProfit
Operating expenses
Operations and maintenance	106,561	103,499
Depreciation and amortization	31,630	31,460
General taxes	15,782us-gaap_UtilitiesOperatingExpenseTaxes	15,524us-gaap_UtilitiesOperatingExpenseTaxes
Total operating expenses	153,973us-gaap_UtilitiesOperatingExpense	150,483us-gaap_UtilitiesOperatingExpense
Operating income	109,005us-gaap_OperatingIncomeLoss	109,353us-gaap_OperatingIncomeLoss
Other income	813us-gaap_OtherNonoperatingIncome	633us-gaap_OtherNonoperatingIncome
Other expense	(454)us-gaap_OtherNonoperatingExpense	(1,148)us-gaap_OtherNonoperatingExpense
Interest expense	(11,169)us-gaap_InterestExpense	(12,950)us-gaap_InterestExpense
Income before income taxes	98,195	95,888
Income taxes	(37,814)us-gaap_IncomeTaxExpenseBenefit	(36,812)us-gaap_IncomeTaxExpenseBenefit
Net income	$ 60,381us-gaap_NetIncomeLoss	$ 59,076us-gaap_NetIncomeLoss
Earnings per share (Note 6)
Basic	$ 1.15us-gaap_EarningsPerShareBasic	$ 1.13us-gaap_EarningsPerShareBasic
Diluted	$ 1.13us-gaap_EarningsPerShareDiluted	$ 1.13us-gaap_EarningsPerShareDiluted
Average shares (thousands)
Basic	52,707	52,334
Diluted	53,446	52,512
Dividends declared per share of stock	$ 0.30	$ 0.00

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