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EARNINGS PER SHARE (Details) (USD $)
In Thousands, except Share data, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Jun. 30, 2015
Dec. 31, 2014
Jan. 31, 2014
EARNINGS PER SHARE [Line Items]          
Common stock, shares issued 52,590,112us-gaap_CommonStockSharesIssued     52,083,859us-gaap_CommonStockSharesIssued 51,941,236us-gaap_CommonStockSharesIssued
Basic EPS Calculation          
Net income available for common stock $ 60,381us-gaap_NetIncomeLoss $ 59,076us-gaap_NetIncomeLoss      
Weighted Average Number of Shares Outstanding, Basic 52,707,000us-gaap_WeightedAverageNumberOfSharesOutstandingBasic 52,334,000us-gaap_WeightedAverageNumberOfSharesOutstandingBasic      
Earnings Per Share, Basic $ 1.15us-gaap_EarningsPerShareBasic $ 1.13us-gaap_EarningsPerShareBasic      
Diluted EPS Calculation          
Net income available for common stock 60,381us-gaap_NetIncomeLoss 59,076us-gaap_NetIncomeLoss      
Effect of dilutive securities on income $ 0us-gaap_DilutiveSecurities $ 0us-gaap_DilutiveSecurities      
Effect of dilutive securities on shares 739,000us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment 178,000us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment      
Weighted Average Number of Shares Outstanding, Diluted 53,446,000us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding 52,512,000us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding      
Earnings Per Share, Diluted $ 1.13us-gaap_EarningsPerShareDiluted $ 1.13us-gaap_EarningsPerShareDiluted      
Dividend Declared [Member]          
Dividends          
Common Stock, Dividends, Per Share, Declared     $ 0.30us-gaap_CommonStockDividendsPerShareDeclared
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Common Stock, Dividends, Declared, Annualized Basis     $ 1.20ogs_CommonStockDividendsDeclaredAnnualizedBasis
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