Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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Aggregate number of common shares reserved for future issuance.

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Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Unrecognized cost of unvested share-based awards, other than options, awarded to employees as compensation.

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The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the period.

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Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

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Equity - Stock-Based Compensation (Narrative) (Details) - USD ($)	1 Months Ended	2 Months Ended	3 Months Ended		9 Months Ended		10 Months Ended	12 Months Ended
	Oct. 31, 2018	Feb. 28, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Jun. 27, 2018	Jun. 15, 2017	Feb. 27, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, shares reserved for future issuance (in shares)											7,080,000
Tax benefit from compensation expense					$ 600,000
Common shares vested (in shares)					210,400
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period					3 years		3 years
Unrecognized compensation costs			$ 6,200,000		$ 6,200,000		$ 6,000,000
Unrecognized compensation cost, weighted average period of recognition					2 years		3 years
Common shares vested (in shares)					210,000		3,000
Performance-based Restricted Stock Units (PRSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period							3 years
Unrecognized compensation costs			3,400,000		$ 3,400,000		$ 3,500,000
Unrecognized compensation cost, weighted average period of recognition					2 years		3 years
Common shares vested (in shares)					0		0
Lease operating expenses
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share-based compensation expense		$ 0	100,000	$ 0	$ 100,000	$ 0	$ 30,000	$ 0
General and administrative expense
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated share-based compensation expense		$ 0	$ 1,100,000	$ 900,000	$ 3,400,000	$ 900,000	$ 1,800,000	$ 0
Restated Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, shares reserved for future issuance (in shares)									10,000,000
Number of shares available for grant			8,400,000		8,400,000
2017 Omnibus Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, shares reserved for future issuance (in shares)										6,876,500
Subsequent Event \| Restated Incentive Plan \| Performance-based Restricted Stock Units (PRSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common shares vested (in shares)	454,000