CONSOLIDATED BALANCE SHEETS (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Current Assets
Cash and cash equivalents	$ 1,160.1us-gaap_CashEquivalentsAtCarryingValue	$ 11.9us-gaap_CashEquivalentsAtCarryingValue
Accounts receivable, net	441.9us-gaap_AccountsReceivableNetCurrent	330.3us-gaap_AccountsReceivableNetCurrent
Fair value of derivative contracts	339.0us-gaap_DerivativeInstrumentsAndHedges	0.2us-gaap_DerivativeInstrumentsAndHedges
Gas, oil and NGL inventories, at lower of average cost or market	13.7us-gaap_EnergyRelatedInventory	13.4us-gaap_EnergyRelatedInventory
Deferred income taxes - current	0	27.9
Prepaid expenses and other	46.8	45.4
Current assets of discontinued operations	0	122.0
Total Current Assets	2,001.5us-gaap_AssetsCurrent	551.1us-gaap_AssetsCurrent
Property, Plant and Equipment (successful efforts method for gas and oil properties)
Proved properties	12,278.7	11,571.4
Unproved properties	825.2	665.1
Marketing and other	293.8us-gaap_PropertyPlantAndEquipmentOther	282.8us-gaap_PropertyPlantAndEquipmentOther
Materials and supplies	54.3us-gaap_InventoryDrillingNoncurrent	54.3us-gaap_InventoryDrillingNoncurrent
Total Property, Plant and Equipment	13,452.0us-gaap_PropertyPlantAndEquipmentGross	12,573.6us-gaap_PropertyPlantAndEquipmentGross
Less Accumulated Depreciation, Depletion and Amortization
Exploration and production	6,153.0	4,930.9
Marketing and other	67.8	50.2
Total Accumulated Depreciation, Depletion and Amortization	6,220.8	4,981.1
Net Property, Plant and Equipment	7,231.2us-gaap_PropertyPlantAndEquipmentNet	7,592.5us-gaap_PropertyPlantAndEquipmentNet
Fair value of derivative contracts	9.9	1.0
Restricted cash	0	50.0
Other noncurrent assets	44.2us-gaap_OtherAssetsNoncurrent	46.6us-gaap_OtherAssetsNoncurrent
Noncurrent assets of discontinued operations	0	1,167.7
TOTAL ASSETS	9,286.8us-gaap_Assets	9,408.9us-gaap_Assets
Current Liabilities
Checks outstanding in excess of cash balances	54.7us-gaap_BankOverdrafts	109.1us-gaap_BankOverdrafts
Accounts payable and accrued expenses	575.4	361.9
Income taxes payable	532.1us-gaap_AccruedIncomeTaxesCurrent	8.7us-gaap_AccruedIncomeTaxesCurrent
Production and property taxes	61.7	54.7
Interest payable	36.4us-gaap_InterestPayableCurrent	37.2us-gaap_InterestPayableCurrent
Fair value of derivative contracts	0	26.7
Deferred income taxes	84.5us-gaap_DeferredTaxLiabilitiesCurrent	0us-gaap_DeferredTaxLiabilitiesCurrent
Current liabilities of discontinued operations	0	75.3
Total Current Liabilities	1,344.8us-gaap_LiabilitiesCurrent	673.6us-gaap_LiabilitiesCurrent
Long-term debt	2,218.1us-gaap_LongTermDebtNoncurrent	2,997.5us-gaap_LongTermDebtNoncurrent
Deferred income taxes	1,362.7	1,364.9
Asset retirement obligations	193.8	163.3
Other long-term liabilities	92.1us-gaap_OtherLiabilitiesNoncurrent	94.5us-gaap_OtherLiabilitiesNoncurrent
Noncurrent liabilities of discontinued operations	0	238.3
Commitments and contingencies (Note 10)
EQUITY
Common stock - par value $0.01 per share; 500.0 million shares authorized; 176.2 million and 179.7 million shares issued, respectively	1.8us-gaap_CommonStockValue	1.8us-gaap_CommonStockValue
Treasury stock - 0.8 million and 0.4 million shares, respectively	(25.4)us-gaap_TreasuryStockValue	(14.9)us-gaap_TreasuryStockValue
Additional paid-in capital	535.3	498.4
Retained earnings	3,587.9	2,917.8
Accumulated other comprehensive (loss) income	(24.3)	(26.5)
Total Common Shareholders' Equity	4,075.3us-gaap_StockholdersEquity	3,376.6us-gaap_StockholdersEquity
Noncontrolling interest	0us-gaap_MinorityInterest	500.2us-gaap_MinorityInterest
Total Equity	4,075.3	3,876.8
TOTAL LIABILITIES AND EQUITY	$ 9,286.8	$ 9,408.9

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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Carrying value as of the balance sheet date of [accrued] interest payable on all forms of debt, including trade payables, that has been incurred and is unpaid. Used to reflect the current portion of the liabilities (due within one year or within the normal operating cycle if longer).

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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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