CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
OPERATING ACTIVITIES
Net (loss) income	$ 217.2	$ 154.1
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation, depletion and amortization	757.1	648.3
Deferred income taxes	39.3	54.7
Impairment	4.0	70.9
Share-based compensation	20.0	19.5
Amortization of debt issuance costs and discounts	4.7	3.7
Net gain from asset sales	(113.0)	(1.5)
Income from unconsolidated affiliates	(3.7)	(5.6)
Distributions from unconsolidated affiliates and other	5.9	6.1
Unrealized loss (gain) on derivative contracts	55.5	(32.8)
Changes in operating assets and liabilities	(8.1)	54.6
Net Cash Provided by Operating Activities	978.9	972.0
INVESTING ACTIVITIES
Property acquisitions	(39.3)	(1,400.3)
Property, plant and equipment, including dry exploratory well expense	(1,089.6)	(1,040.7)
Proceeds from disposition of assets	208.3	5.3
Net Cash Used in Investing Activities	(920.6)	(2,435.7)
FINANCING ACTIVITIES
Checks outstanding in excess of cash balances	(38.1)	(1.9)
Long-term debt issued	0	1,450.0
Long-term debt issuance costs paid	(3.0)	(17.0)
Long-term debt repaid	0	6.7
Proceeds from credit facility	1,075.0	933.5
Repayments of credit facility	(1,400.0)	(876.0)
Payments for Repurchase of Common Stock	(8.7)	0
Proceeds from Issuance of Common Limited Partners Units	449.6	0
Other capital contributions	3.6	(4.2)
Dividends paid	(10.8)	(10.7)
Excess tax benefit on share-based compensation	1.3	2.0
Distribution to noncontrolling interest	(4.2)	(5.3)
Net Cash Provided by Financing Activities	64.7	1,463.7
Change in cash and cash equivalents	123.0	0
Beginning cash and cash equivalents	0	0
Ending cash and cash equivalents	123.0	0
Supplemental Disclosures:
Cash paid for interest, net of capitalized interest	120.1	79.8
Cash paid (received) for income taxes	49.4	28.0
Non-cash investing activities
Change in capital expenditure accrual balance	$ 53.7	$ 97.5