CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
OPERATING ACTIVITIES
Net income (loss)	$ (1,245.0)	$ (149.4)	$ 784.4
Net income attributable to noncontrolling interest	0.0	0.0	21.6
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation, depletion and amortization	871.1	881.1	1,040.6
Deferred income taxes	(651.3)	25.3	(84.1)
Impairment	1,194.3	55.6	1,143.2
Bargain purchase gain from acquisition	(22.6)	0.0	0.0
Share-based compensation	35.6	34.7	27.9
Pension curtailment loss	0.0	11.2	0.0
Amortization of debt issuance costs and discounts	6.4	6.2	6.7
Net gain from asset sales	(5.0)	(4.6)	(1,644.8)
Income from unconsolidated affiliates	0.0	0.0	(5.2)
Distributions from unconsolidated affiliates and other	0.0	0.0	9.4
Non-cash loss on early extinguishment of debt	0.0	0.0	4.4
Unrealized (gains) losses on marketable securities	(1.4)	0.2	0.0
Unrealized gains (losses) on derivative contracts	367.0	183.7	(374.4)
Accounts receivable	96.5	165.5	(160.5)
Inventories	8.7	15.5	(20.2)
Prepaid expenses	18.5	16.7	(7.3)
Accounts payable and accrued expenses	(51.5)	(71.3)	320.1
Federal income taxes	68.7	(619.4)	494.1
Other	(26.3)	(69.7)	(13.4)
Net Cash Provided by (Used in) Operating Activities	663.7	481.3	1,542.5
INVESTING ACTIVITIES
Property acquisitions	(639.0)	(98.3)	(960.5)
Property, plant and equipment, including dry exploratory well expense	(569.1)	(1,141.1)	(1,765.9)
Proceeds from disposition of assets	29.0	21.8	3,296.6
Acquisition deposit held in escrow	0.0	0.0	50.0
Other investing activities	0.0	0.0	(42.0)
Net Cash Provided by (Used in) Investing Activities	(1,179.1)	(1,217.6)	578.2
FINANCING ACTIVITIES
Checks outstanding in excess of cash balances	(17.5)	(24.9)	(54.4)
Long-term debt issued	0.0	0.0	300.0
Long-term debt issuance costs paid	0.0	(2.6)	(9.3)
Long-term debt repaid	(176.8)	0.0	(600.0)
Proceeds from credit facility	0.0	0.0	5,455.0
Repayments of credit facility	0.0	0.0	(5,935.0)
Common stock repurchased and retired	0.0	0.0	(99.7)
Treasury stock repurchases	(4.1)	(2.7)	(6.2)
Other capital contributions	0.0	(0.2)	6.0
Dividends paid	0.0	(14.1)	(14.6)
Proceeds from issuance of common stock, net	781.4	0.0	0.0
Excess tax (provision) benefit on share-based compensation	0.1	(3.2)	(0.5)
Distribution to noncontrolling interest	0.0	0.0	(31.9)
Net Cash Provided by (Used in) Financing Activities	583.1	(47.7)	(990.6)
Change in cash and cash equivalents	67.7	(784.0)	1,130.1
Beginning cash and cash equivalents	376.1	1,160.1	30.0
Ending cash and cash equivalents	$ 443.8	$ 376.1	$ 1,160.1