CONDENSED CONSOLIDATED BALANCE SHEETS (Unaudited) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Current Assets
Cash and cash equivalents	$ 500.4	$ 1,160.1
Accounts receivable, net	319.4us-gaap_AccountsReceivableNetCurrent	441.9us-gaap_AccountsReceivableNetCurrent
Fair value of derivative contracts	311.7us-gaap_DerivativeInstrumentsAndHedges	339.0us-gaap_DerivativeInstrumentsAndHedges
Gas, oil and NGL inventories at lower of average cost or market	7.6us-gaap_EnergyRelatedInventory	13.7us-gaap_EnergyRelatedInventory
Prepaid expenses and other	37.8	46.8
Total Current Assets	1,176.9us-gaap_AssetsCurrent	2,001.5us-gaap_AssetsCurrent
Property, Plant and Equipment (successful efforts method for gas and oil properties)
Proved properties	12,558.9	12,278.7
Unproved properties	821.9	825.2
Marketing and resources	296.3us-gaap_PropertyPlantAndEquipmentOther	293.8us-gaap_PropertyPlantAndEquipmentOther
Material and supplies	50.4us-gaap_InventoryDrillingNoncurrent	54.3us-gaap_InventoryDrillingNoncurrent
Total Property, Plant and Equipment	13,727.5us-gaap_PropertyPlantAndEquipmentGross	13,452.0us-gaap_PropertyPlantAndEquipmentGross
Less Accumulated Depreciation, Depletion and Amortization
Exploration and production	6,357.7	6,153.0
Marketing and other	72.8	67.8
Total Accumulated Depreciation, Depletion and Amortization	6,430.5	6,220.8
Net Property, Plant and Equipment	7,297.0us-gaap_PropertyPlantAndEquipmentNet	7,231.2us-gaap_PropertyPlantAndEquipmentNet
Fair value of derivative contracts	13.7	9.9
Other noncurrent assets	36.8us-gaap_OtherAssetsNoncurrent	44.2us-gaap_OtherAssetsNoncurrent
TOTAL ASSETS	8,524.4us-gaap_Assets	9,286.8us-gaap_Assets
Current Liabilities
Checks outstanding in excess of cash balances	15.8us-gaap_BankOverdrafts	54.7us-gaap_BankOverdrafts
Accounts payable and accrued expenses	453.5	575.4
Income taxes payable	0us-gaap_AccruedIncomeTaxesCurrent	532.1us-gaap_AccruedIncomeTaxesCurrent
Production and property taxes	45.9	61.7
Interest payable	33.7us-gaap_InterestPayableCurrent	36.4us-gaap_InterestPayableCurrent
Deferred income taxes	94.0us-gaap_DeferredTaxLiabilitiesCurrent	84.5us-gaap_DeferredTaxLiabilitiesCurrent
Total Current Liabilities	642.9us-gaap_LiabilitiesCurrent	1,344.8us-gaap_LiabilitiesCurrent
Long-term debt	2,218.3us-gaap_LongTermDebtNoncurrent	2,218.1us-gaap_LongTermDebtNoncurrent
Deferred income taxes	1,348.9	1,362.7
Asset retirement obligations	196.3	193.8
Other long-term liabilities	98.1us-gaap_OtherLiabilitiesNoncurrent	92.1us-gaap_OtherLiabilitiesNoncurrent
Commitments and contingencies (Note 11)
EQUITY
Common stock - par value $0.01 per share; 500.0 million shares authorized; 177.0 million and 176.2 million shares issued, respectively	1.8us-gaap_CommonStockValue	1.8us-gaap_CommonStockValue
Treasury stock - 0.3 million and 0.8 million shares, respectively	(11.0)us-gaap_TreasuryStockValue	(25.4)us-gaap_TreasuryStockValue
Additional paid-in capital	530.1	535.3
Retained earnings	3,522.6	3,587.9
Accumulated other comprehensive loss	(23.6)	(24.3)
Total Common Shareholders' Equity	4,019.9us-gaap_StockholdersEquity	4,075.3us-gaap_StockholdersEquity
TOTAL LIABILITIES AND EQUITY	$ 8,524.4	$ 9,286.8

Sum of the carrying values as of the balance sheet date of obligations incurred through that date and due within one year (or the operating cycle, if longer), including liabilities incurred (and for which invoices have typically been received) and payable to vendors for goods and services received, taxes, interest, rent and utilities, accrued salaries and bonuses, payroll taxes and fringe benefits.

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Oil and Gas properties with proved reserves under the successful effort method.

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The amount allocated to treasury stock. Treasury stock is common and preferred shares of an entity that were issued, repurchased by the entity, and are held in its treasury.

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Oil and Gas properties with unproved reserves under the successful effort method.

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