FAIR VALUE MEASUREMENTS (Details 2) (USD $)	Mar. 31, 2013		Dec. 31, 2012
Fair value and carrying value of financial instruments
Derivative instruments assets	$ 13,874,000		$ 48,525,000
Derivative instruments liabilities	20,212,000		16,551,000
Credit Facility	140,000,000		65,000,000
Credit Facility, fair value	140,000,000	[1]	65,000,000	[1]
8 1/2% Senior Notes due 2014
Fair value and carrying value of financial instruments
Notes payable, carrying value			296,723,000
Notes payable, fair value			321,000,000	[1]
Interest rate (as a percent)	8.50%		8.50%
7 1/4% Senior Notes due 2019
Fair value and carrying value of financial instruments
Notes payable, carrying value	1,000,351,000		1,000,365,000
Notes payable, fair value	1,022,000,000	[1]	1,006,850,000	[1]
Interest rate (as a percent)	7.25%		7.25%
7 1/2% Senior Notes due 2020
Fair value and carrying value of financial instruments
Notes payable, carrying value	500,000,000		500,000,000
Notes payable, fair value	529,375,000	[1]	526,250,000	[1]
Interest rate (as a percent)	7.50%		7.50%
Carrying Amount
Fair value and carrying value of financial instruments
Derivative instruments assets	13,874,000		48,525,000
Derivative instruments liabilities	20,212,000		16,551,000
Fair Value
Fair value and carrying value of financial instruments
Derivative instruments assets	13,874,000	[1]	48,525,000	[1]
Derivative instruments liabilities	20,212,000	[1]	16,551,000	[1]
Using Quoted Prices in Active Markets for Identical Assets (Level 1) | 8 1/2% Senior Notes due 2014
Fair value and carrying value of financial instruments
Notes payable, fair value			321,000,000
Using Quoted Prices in Active Markets for Identical Assets (Level 1) | 7 1/4% Senior Notes due 2019
Fair value and carrying value of financial instruments
Notes payable, fair value	1,022,000,000		1,006,850,000
Using Quoted Prices in Active Markets for Identical Assets (Level 1) | 7 1/2% Senior Notes due 2020
Fair value and carrying value of financial instruments
Notes payable, fair value	529,375,000		526,250,000
Using Significant Other Observable Inputs (Level 2)
Fair value and carrying value of financial instruments
Credit Facility, fair value	140,000,000		65,000,000
Using Significant Other Observable Inputs (Level 2) | Fair Value
Fair value and carrying value of financial instruments
Derivative instruments assets	13,874,000		48,525,000
Derivative instruments liabilities	$ 20,212,000		$ 16,551,000

[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.