STOCK-BASED COMPENSATION (Details) (USD $)	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012		Dec. 31, 2012
Stock-based compensation
Total stock-based compensation costs	$ 7,125,000	[1],[2]	$ 5,921,000	[1]
Less: stock-based compensation costs capitalized	(2,964,000)	[1],[2]	(2,597,000)	[1]
Stock-based compensation costs expensed	4,161,000	[1],[2]	3,324,000	[1]
Unamortized stock-based compensation costs	36,199,000	[1],[2]
Weighted average amortization period remaining	1 year 9 months 18 days	[1],[2]
Stock option activity
Options outstanding at beginning of period (in shares)	870,816
Granted (in shares)	0
Exercised (in shares)	0
Cancelled (in shares)	(42,476)
Options outstanding at end of period (in shares)	828,340
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)	$ 11.02
Options outstanding, end of period (in dollars per share)	$ 18.21
Stock Options, Aggregate Intrinsic Value
Aggregate intrinsic value for stock options at beginning of the period	0	[3]
Aggregate intrinsic value for stock options exercised	0	[3]
Aggregate intrinsic value for stock options at end of the period	0	[3]
Stock Options, Number of Options Exercisable
Options outstanding, exercisable number (in shares)	828,340				870,816
One-time employee termination benefit costs
Stock-based compensation
Total stock-based compensation costs	2,000,000	[1],[2]
Stock-based compensation costs expensed	1,000,000	[1],[2]
One-time employee termination benefit costs	7,500,000
One-time employee termination benefit costs, net of capitalized amounts	5,700,000
Restricted Stock
Stock-based compensation
Total stock-based compensation costs	4,235,000		3,776,000
Less: stock-based compensation costs capitalized	(1,822,000)		(1,708,000)
Stock-based compensation costs expensed	2,413,000		2,068,000
Unamortized stock-based compensation costs	19,905,000
Weighted average amortization period remaining	1 year 9 months 18 days
Share-based compensation plan activity
Unvested at beginning of period (in shares)	2,721,637
Awarded (in shares)	1,231,851	[4]
Vested (in shares)	(220,475)
Forfeited (in shares)	(127,250)
Unvested at end of period (in shares)	3,605,763
Weighted Average Grant Date Fair Value
Unvested at beginning of period (in dollars per share)	$ 17.64
Awarded (in dollars per share)	$ 6.83
Vested (in dollars per share)	$ 19.61
Forfeited (in dollars per share)	$ 16.71
Unvested at end of period (in dollars per share)	$ 13.86
Vest Date Fair Value
Vested, Vest Date Fair Value	1,479,000
Restricted Stock | Vesting Schedule One-Third On Each Of the First Three Anniversary Dates of the Grant
Share-based compensation plan activity
Awarded (in shares)	1,229,901
Vest Date Fair Value
Vesting rate per anniversary date of grant (as a percent)	33.30%
Award vesting period (in years)	3 years
Performance Units
Stock-based compensation
Total stock-based compensation costs	1,628,000		1,212,000
Less: stock-based compensation costs capitalized	(473,000)		(390,000)
Stock-based compensation costs expensed	1,155,000		822,000
Unamortized stock-based compensation costs	6,188,000
Weighted average amortization period remaining	1 year 7 months 6 days
Share-based compensation plan activity
Unvested at beginning of period (in shares)	939,180
Awarded (in shares)	0
Vested (in shares)	(68,720)
Forfeited (in shares)	0
Unvested at end of period (in shares)	870,460
Weighted Average Grant Date Fair Value
Unvested at beginning of period (in dollars per share)	$ 15.20
Awarded (in dollars per share)	$ 0.00
Vested (in dollars per share)	$ 17.22
Forfeited (in dollars per share)	$ 0.00
Unvested at end of period (in dollars per share)	$ 15.04
Vest Date Fair Value
Vested, Vest Date Fair Value	0
Phantom Stock Units
Stock-based compensation
Total stock-based compensation costs	1,262,000		933,000
Less: stock-based compensation costs capitalized	(669,000)		(499,000)
Stock-based compensation costs expensed	593,000		434,000
Unamortized stock-based compensation costs	10,106,000	[5]
Weighted average amortization period remaining	2 years
Share-based compensation plan activity
Unvested at beginning of period (in shares)	1,161,737	[6]
Awarded (in shares)	1,249,901	[4],[6]
Vested (in shares)	(90,821)	[6]
Forfeited (in shares)	(63,586)	[6]
Unvested at end of period (in shares)	2,257,231	[6]
Weighted Average Grant Date Fair Value
Unvested at beginning of period (in dollars per share)	$ 9.91
Awarded (in dollars per share)	$ 6.84
Vested (in dollars per share)	$ 10.49
Forfeited (in dollars per share)	$ 10.38
Unvested at end of period (in dollars per share)	$ 8.17
Vest Date Fair Value
Vested, Vest Date Fair Value	615,000
Phantom Stock Units | Vesting Schedule One-Third On Each Of the First Three Anniversary Dates of the Grant
Share-based compensation plan activity
Awarded (in shares)	1,249,901	[4]
Vest Date Fair Value
Vesting rate per anniversary date of grant (as a percent)	33.30%
Award vesting period (in years)	3 years
Employee Stock Purchase Plan
Stock-based compensation
Total stock-based compensation costs	$ 100,000		$ 100,000

[1] Forest also maintains an employee stock purchase plan (which is not included in the table) under which $.1 million of compensation cost was recognized for each of the three month periods ended March 31, 2013 and 2012.

[2] In connection with the divestiture of the South Texas oil and natural gas properties during the three months ended March 31, 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, during the three months ended March 31, 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 three months ended March 31, 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 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.

[4] Forest granted 1,229,901 shares of restricted stock and 1,249,901 phantom stock units during the three months ended March 31, 2013 that vest in increments of 33.3% on each of the first three anniversary dates of the grant.

[5] The unamortized stock-based compensation costs of the phantom stock units are based on the closing price of Forest’s common stock at the reporting period end.

[6] All of the unvested units of phantom stock at March 31, 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 three months ended March 31, 2013 were settled in cash.