STOCK-BASED COMPENSATION (Details) (USD $)	3 Months Ended				9 Months Ended					3 Months Ended					9 Months Ended				3 Months Ended			9 Months Ended				3 Months Ended			9 Months Ended			3 Months Ended		9 Months Ended
	Sep. 30, 2013		Sep. 30, 2012		Sep. 30, 2013		Sep. 30, 2012		Dec. 31, 2012	Mar. 31, 2013 One-time employee termination benefit costs		Sep. 30, 2013 Restricted Stock		Sep. 30, 2012 Restricted Stock	Sep. 30, 2013 Restricted Stock		Sep. 30, 2012 Restricted Stock	Sep. 30, 2013 Restricted Stock Vesting Schedule One-Third On Each Of the First Three Anniversary Dates of the Grant	Sep. 30, 2013 Performance Units		Sep. 30, 2012 Performance Units	Sep. 30, 2013 Performance Units		Sep. 30, 2012 Performance Units	Sep. 30, 2013 Performance Units Cash-settled	Sep. 30, 2013 Phantom Stock Units		Sep. 30, 2012 Phantom Stock Units	Sep. 30, 2013 Phantom Stock Units		Sep. 30, 2012 Phantom Stock Units	Sep. 30, 2013 Employee Stock Purchase Plan	Sep. 30, 2012 Employee Stock Purchase Plan	Sep. 30, 2013 Employee Stock Purchase Plan	Sep. 30, 2012 Employee Stock Purchase Plan
Stock-based compensation
Total stock-based compensation costs	$ 3,707,000	[1]	$ 5,682,000	[1]	$ 15,660,000	[1],[2]	$ 18,645,000	[1]		$ 2,000,000	[1],[2]	$ 1,644,000		$ 3,500,000	$ 9,089,000		$ 12,219,000		$ 301,000		$ 1,273,000	$ 3,034,000		$ 5,630,000		$ 1,762,000		$ 909,000	$ 3,537,000		$ 796,000	$ 40,000	$ 100,000	$ 200,000	$ 300,000
Less: stock-based compensation costs capitalized	(1,950,000)	[1]	(2,292,000)	[1]	(6,545,000)	[1],[2]	(6,484,000)	[1]				(877,000)		(1,435,000)	(3,871,000)		(4,630,000)		(245,000)		(455,000)	(959,000)		(1,322,000)		(828,000)		(402,000)	(1,715,000)		(532,000)
Stock-based compensation costs expensed	1,757,000	[1]	3,390,000	[1]	9,115,000	[1],[2]	12,161,000	[1]		1,000,000	[1],[2]	767,000		2,065,000	5,218,000		7,589,000		56,000		818,000	2,075,000		4,308,000		934,000		507,000	1,822,000		264,000
Unamortized stock-based compensation costs	35,688,000	[1],[2],[3]			35,688,000	[1],[2],[3]						15,541,000	[3]		15,541,000	[3]			8,647,000	[3]		8,647,000	[3]			11,500,000	[3]		11,500,000	[3]
Weighted average amortization period remaining					1 year 9 months 18 days	[1],[2]									1 year 7 months 6 days							2 years 1 month 6 days							1 year 10 months 24 days
One-time employee termination benefit costs										7,500,000
One-time employee termination benefit costs, net of capitalized amounts										5,700,000
Stock option activity
Options outstanding at beginning of period (in shares)					870,816
Granted (in shares)					0
Exercised (in shares)					0
Cancelled (in shares)					(223,800)
Options outstanding at end of period (in shares)	647,016				647,016
Stock Options, Weighted Average Exercise Price
Options outstanding, beginning of period (in dollars per share)					$ 17.86
Granted (in dollars per share)					$ 0.00
Exercised (in dollars per share)					$ 0.00
Cancelled (in dollars per share)					$ 19.87
Options outstanding, end of period (in dollars per share)	$ 17.16				$ 17.16
Stock Options, Aggregate Intrinsic Value
Aggregate intrinsic value for stock options at beginning of the period					0	[4]
Aggregate intrinsic value for stock options exercised					0	[4]
Aggregate intrinsic value for stock options at end of the period	0	[4]			0	[4]
Stock Options, Number of Options Exercisable
Options outstanding, exercisable number (in shares)	647,016				647,016				870,816
Share-based compensation plan activity
Unvested at beginning of period (in shares)															2,721,637							939,180							1,161,737	[5]
Awarded (in shares)															2,163,877	[6]		1,229,901				1,182,500	[6]		950,000				1,724,701	[5]
Vested (in shares)															(837,894)							(203,240)							(93,044)	[5]
Forfeited (in shares)															(565,900)							(256,500)							(286,904)	[5]
Unvested at end of period (in shares)												3,481,720			3,481,720				1,661,940			1,661,940				2,506,490	[5]		2,506,490	[5]
Weighted Average Grant Date Fair Value
Unvested at beginning of period (in dollars per share)															$ 17.64							$ 15.20							$ 9.91
Awarded (in dollars per share)															$ 6.04							$ 5.47							$ 6.05
Vested (in dollars per share)															$ 21.67							$ 19.60							$ 10.55
Forfeited (in dollars per share)															$ 11.95							$ 9.59							$ 8.65
Unvested at end of period (in dollars per share)												$ 10.38			$ 10.38				$ 8.61			$ 8.61				$ 7.37			$ 7.37
Vest Date Fair Value
Vested, Vest Date Fair Value															$ 4,669,000							$ 0							$ 627,000
Vesting rate per anniversary date of grant (as a percent)																		33.30%
Award vesting period (in years)																		3 years
Performance period for cash settled performance units																									36 months

[1] Forest also maintains an employee stock purchase plan (which is not included in the table) under which $.04 million and $.2 million of compensation cost was recognized for the three and nine month periods ended September 30, 2013, respectively, and $.1 million and $.3 million of compensation cost was recognized for the three and nine month periods ended September 30, 2012, respectively.

[2] In connection with the divestiture of the South Texas oil and natural gas properties in the first quarter of 2013, Forest incurred one-time employee termination benefit costs that included $2.0 million ($1.0 million net of capitalized amounts) in stock-based compensation costs due to accelerated vesting of the affected awards. In addition to these stock-based compensation costs, in the first quarter of 2013, Forest incurred $7.5 million ($5.7 million net of capitalized amounts) in other one-time employee termination benefit costs related to this divestiture. All of these one-time employee termination benefit costs expenses are included in “General and administrative” in the Condensed Consolidated Statement of Operations for the nine months ended September 30, 2013 and no further one-time employee termination benefit costs are expected to be made for this specific divestiture. See Note 5 for more information regarding this divestiture.

[3] The unamortized stock-based compensation costs for liability-based awards are based on the closing price of Forest’s common stock at the reporting period end.

[4] The intrinsic value of a stock option is the amount by which the market value of the underlying stock, as of the date outstanding or exercised, exceeds the exercise price of the option.

[5] All of the unvested phantom stock units at September 30, 2013 must be settled in cash. The phantom stock units have been accounted for as a liability within the Condensed Consolidated Financial Statements. All of the phantom stock units that vested during the nine months ended September 30, 2013 were settled in cash.

[6] Of the restricted stock shares granted during the nine months ended September 30, 2013, 1,229,901 shares vest in increments of 33.3% on each of the first three anniversary dates of the grant. All other Forest restricted stock grants cliff vest on the third anniversary of the date of grant. Additionally, during the nine months ended September 30, 2013, Forest granted 950,000 performance units that are cash-settled, with the settlement amount dependent upon Forest’s relative total shareholder return in comparison to an identified peer group over a thirty-six month performance period. These performance units have been accounted for as a liability within the Condensed Consolidated Financial Statements. All of the other outstanding performance units are share-settled and utilize the same relative total shareholder return criteria to determine the settlement amount, other than certain peers used for comparison purposes. The share-settled performance units have been accounted for as equity-based awards within the Condensed Consolidated Financial Statements.