CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
OPERATING ACTIVITIES
Net income (loss)	$ 76.9	$ (863.8)
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Depreciation, depletion and amortization	191.8	240.0
Deferred income taxes	45.6	(446.7)
Impairment	0.1	1,182.4
Bargain purchase gain from acquisition	0.4	0.0
Share-based compensation	6.0	8.0
Amortization of debt issuance costs and discounts	1.5	1.6
Net (gain) loss from asset sales	0.0	(0.5)
Unrealized (gains) losses on marketable securities	(0.8)	(0.2)
Unrealized (gains) losses on derivative contracts	(177.3)	13.5
Changes in operating assets and liabilities	5.2	(52.6)
Net Cash Provided by (Used in) Operating Activities	149.4	81.7
INVESTING ACTIVITIES
Property acquisitions	(68.2)	(14.8)
Property, plant and equipment, including dry exploratory well expense	(177.3)	(185.8)
Proceeds from disposition of assets	0.2	22.9
Net Cash Provided by (Used in) Investing Activities	(245.3)	(177.7)
FINANCING ACTIVITIES
Checks outstanding in excess of cash balances	(3.1)	(29.8)
Treasury stock repurchases	(6.4)	(2.9)
Other capital contributions	0.0	0.2
Proceeds from issuance of common stock, net	0.0	368.6
Excess tax (provision) benefit on share-based compensation	0.0	0.2
Net Cash Provided by (Used in) Financing Activities	(9.5)	336.3
Change in cash and cash equivalents	(105.4)	240.3
Beginning cash and cash equivalents	443.8	376.1
Ending cash and cash equivalents	338.4	616.4
Supplemental Disclosures:
Cash paid for interest, net of capitalized interest	32.3	37.8
Cash paid (refund received) for income taxes	0.0	32.0
Non-cash Investing Activities
Change in capital expenditure accruals and other non-cash adjustments	$ 37.0	$ (22.6)