CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
OPERATING ACTIVITIES
Net income (loss)	$ (1,011.6)	$ 269.3	$ (1,245.0)
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Depreciation, depletion and amortization	857.1	754.5	871.1
Deferred income taxes	(247.6)	(314.8)	(651.3)
Impairment	1,560.9	78.9	1,194.3
Dry hole exploratory well expense	0.0	21.3	0.0
Share-based compensation	39.1	22.4	35.6
Amortization of debt issuance costs and discounts	5.4	6.2	6.4
Bargain purchase gain from acquisitions	0.0	0.4	(22.6)
Net (gain) loss from asset sales, inclusive of restructuring costs	(25.0)	(213.5)	(5.0)
Loss from early extinguishment of debt	0.0	32.7	0.0
Unrealized (gains) losses on marketable securities	1.2	(2.9)	(1.4)
Unrealized (gains) losses on derivative contracts	(248.5)	(40.0)	367.0
Other non-cash activity	0.0	(9.4)	0.0
Accounts receivable	33.7	(2.0)	95.3
Prepaid expenses	(2.0)	(1.3)	18.7
Accounts payable and accrued expenses	(74.2)	3.5	(50.3)
Income taxes receivable	(71.0)	13.7	68.7
Other	(1.3)	(18.8)	(14.3)
Net Cash Provided by (Used in) Operating Activities	816.2	600.2	667.2
INVESTING ACTIVITIES
Property acquisitions	(65.6)	(815.2)	(639.0)
Property, plant and equipment, including exploratory well expense	(1,234.1)	(1,159.6)	(569.1)
Proceeds from disposition of assets	243.6	806.8	29.0
Net Cash Provided by (Used in) Investing Activities	(1,056.1)	(1,168.0)	(1,179.1)
FINANCING ACTIVITIES
Checks outstanding in excess of cash balances	(29.5)	31.7	(17.5)
Long-term debt issued	0.0	500.0	0.0
Long-term debt issuance costs paid	(0.1)	(14.4)	0.0
Long-term debt extinguishment costs paid	0.0	(28.1)	0.0
Long-term debt repaid	0.0	(445.6)	(176.8)
Proceeds from credit facility	3,608.0	492.0	0.0
Repayments of credit facility	(3,267.0)	(403.0)	0.0
Common stock repurchased and retired	(58.4)	0.0	0.0
Treasury stock repurchases	(8.7)	(6.8)	(4.1)
Other capital contributions	0.3	0.0	0.0
Proceeds from issuance of common stock, net	0.0	0.0	781.4
Excess tax (provision) benefit on share-based compensation	0.0	0.0	0.1
Net Cash Provided by (Used in) Financing Activities	244.6	125.8	583.1
Change in cash, cash equivalents and restricted cash(1)	4.7	(442.0)	71.2
Beginning cash, cash equivalents and restricted cash(1)	23.4	465.4	394.2
Ending cash, cash equivalents and restricted cash(1)	$ 28.1	$ 23.4	$ 465.4