FAIR VALUE MEASUREMENTS (Details 2) (USD $)	Sep. 30, 2013		Dec. 31, 2012		Mar. 31, 2013 8 1/2% Senior Notes due 2014	Dec. 31, 2012 8 1/2% Senior Notes due 2014		Sep. 30, 2013 7 1/4% Senior Notes due 2019		Dec. 31, 2012 7 1/4% Senior Notes due 2019		Sep. 30, 2013 7 1/2% Senior Notes due 2020		Dec. 31, 2012 7 1/2% Senior Notes due 2020		Sep. 30, 2013 Carrying Amount	Dec. 31, 2012 Carrying Amount	Sep. 30, 2013 Fair Value		Dec. 31, 2012 Fair Value		Dec. 31, 2012 Using Quoted Prices in Active Markets for Identical Assets (Level 1) 8 1/2% Senior Notes due 2014	Sep. 30, 2013 Using Quoted Prices in Active Markets for Identical Assets (Level 1) 7 1/4% Senior Notes due 2019	Dec. 31, 2012 Using Quoted Prices in Active Markets for Identical Assets (Level 1) 7 1/4% Senior Notes due 2019	Sep. 30, 2013 Using Quoted Prices in Active Markets for Identical Assets (Level 1) 7 1/2% Senior Notes due 2020	Dec. 31, 2012 Using Quoted Prices in Active Markets for Identical Assets (Level 1) 7 1/2% Senior Notes due 2020	Sep. 30, 2013 Using Significant Other Observable Inputs (Level 2)	Dec. 31, 2012 Using Significant Other Observable Inputs (Level 2)	Sep. 30, 2013 Using Significant Other Observable Inputs (Level 2) Fair Value	Dec. 31, 2012 Using Significant Other Observable Inputs (Level 2) Fair Value
Fair value and carrying value of financial instruments
Derivative instruments assets	$ 19,500,000		$ 48,525,000													$ 19,500,000	$ 48,525,000	$ 19,500,000	[1]	$ 48,525,000	[1]								$ 19,500,000	$ 48,525,000
Derivative instruments liabilities	9,601,000		16,551,000													9,601,000	16,551,000	9,601,000	[1]	16,551,000	[1]								9,601,000	16,551,000
Credit Facility	115,000,000		65,000,000
Credit Facility, fair value	115,000,000	[1]	65,000,000	[1]																							115,000,000	65,000,000
Notes payable, carrying value						296,723,000		1,000,322,000		1,000,365,000		500,000,000		500,000,000
Notes payable, fair value						$ 321,000,000	[1]	$ 1,017,000,000	[1]	$ 1,006,850,000	[1]	$ 498,125,000	[1]	$ 526,250,000	[1]							$ 321,000,000	$ 1,017,000,000	$ 1,006,850,000	$ 498,125,000	$ 526,250,000
Interest rate (as a percent)					8.50%	8.50%		7.25%		7.25%		7.50%		7.50%

[1] Forest used various assumptions and methods in estimating the fair values of its financial instruments. The fair values of the senior notes were estimated based on quoted market prices. The carrying amount of the Credit Facility approximated fair value due to the short original maturities of the borrowings and because the borrowings bear interest at variable market rates. The methods used to determine the fair values of the derivative instruments are discussed above. See also Note 8 for more information on the derivative instruments.