CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
OPERATING ACTIVITIES
Net income (loss)	$ (53.6)	$ 76.9
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Depreciation, depletion and amortization	196.5	191.8
Deferred income taxes (benefit)	(14.1)	45.6
Impairment	0.7	0.1
Share-based compensation	11.2	6.0
Amortization of debt issuance costs and discounts	1.3	1.5
Bargain purchase gain from acquisition	0.0	0.4
Net gain (loss) from asset sales	(3.5)	0.0
Unrealized (gains) losses on marketable securities	0.1	(0.8)
Unrealized (gains) losses on derivative contracts	10.0	(177.3)
Changes in operating assets and liabilities	11.8	5.7
Net Cash Provided by (Used in) Operating Activities	160.4	149.9
INVESTING ACTIVITIES
Property acquisitions	(36.2)	(68.2)
Property, plant and equipment, including exploratory well expense	(370.7)	(177.3)
Proceeds from disposition of assets	33.3	0.2
Net Cash Provided by (Used in) Investing Activities	(373.6)	(245.3)
FINANCING ACTIVITIES
Checks outstanding in excess of cash balances	(24.2)	(3.1)
Proceeds from credit facility	1,068.5	0.0
Repayments of credit facility	(772.5)	0.0
Common stock repurchased and retired	(52.8)	0.0
Treasury stock repurchases	(4.7)	(6.4)
Net Cash Provided by (Used in) Financing Activities	214.3	(9.5)
Change in cash, cash equivalents and restricted cash	1.1	(104.9)
Beginning cash, cash equivalents and restricted cash(1)	23.4	465.4
Ending cash, cash equivalents and restricted cash(1)	24.5	360.5
Supplemental Disclosures:
Cash paid for interest, net of capitalized interest	26.0	32.3
Cash paid for income taxes, net	0.0	0.0
Non-cash Investing Activities:
Change in capital expenditure accruals and other non-cash adjustments	$ 48.1	$ 37.0