Debt and Banking Arrangements - Debt - Additional Information (Detail) $ in Millions	3 Months Ended			12 Months Ended
	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jun. 30, 2019 USD ($)
Debt Instrument [Line Items]
Interest Paid, Including Capitalized Interest, Operating and Investing Activities				$ 150.0	$ 172.0	$ 178.0
Write off of Deferred Debt Issuance Cost	$ 3.0		$ 6.0
Gain (Loss) on Extinguishment of Debt, before Write off of Debt Issuance Cost	(44.0)		(63.0)
Debt Instrument, Repurchase Amount	550.0		921.0
Maximum Limit On Consolidated Secure Indebtedness to Consolidated EBITDAX				4.25
Minimum Current Ratio				1.0
Gain (Loss) on Extinguishment of Debt	(47.0)	$ (71.0)	(71.0)	$ (47.0)	(71.0)	$ (17.0)
Percentage of repurchase of notes on principal amount of notes				101.00%
Total debt				$ 2,229.0	2,509.0
Letters of credit issued				28.0
Credit Facility Agreement
Debt Instrument [Line Items]
Total debt				$ 0.0	$ 330.0
7.500% Senior Notes due 2020
Debt Instrument [Line Items]
Debt Instrument, Repurchase Amount			350.0
Debt instrument stated interest rate				7.50%	7.50%
6.000% Senior Notes due 2022
Debt Instrument [Line Items]
Debt Instrument, Repurchase Amount			571.0
Debt Instrument, Face Amount				$ 73.0
Debt instrument stated interest rate				6.00%	6.00%
Total debt				$ 73.0	$ 529.0
8.250% Senior Notes due 2023
Debt Instrument [Line Items]
Debt Instrument, Face Amount				$ 406.0
Debt instrument stated interest rate				8.25%	8.25%
Total debt				$ 406.0	$ 500.0
5.250 % Senior Notes due 2024
Debt Instrument [Line Items]
Debt Instrument, Face Amount				$ 650.0
Debt instrument stated interest rate				5.25%	5.25%
Total debt				$ 650.0	$ 650.0
5.750% Senior Notes Due 2026
Debt Instrument [Line Items]
Debt Issuance Costs, Gross			1.0
Proceeds from Issuance of Debt			494.0
Debt Instrument, Face Amount			$ 500.0	$ 500.0
Debt instrument stated interest rate			5.75%	5.75%	5.75%
Total debt				$ 500.0	$ 500.0
5.250% Senior Notes due 2027
Debt Instrument [Line Items]
Debt Issuance Costs, Gross	2.0
Proceeds from Issuance of Debt	592.5
Debt Instrument, Face Amount	$ 600.0			$ 600.0
Debt instrument stated interest rate	5.25%			5.25%
Total debt				$ 600.0	$ 0.0
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity				$ 1,500.0
Line of Credit Facility, Capacity Available for Specific Purpose Other than for Trade Purchases							$ 2,100.0