FAIR VALUE MEASUREMENTS - Summary of Assets and liabilities measured at fair value on recurring basis (Details) (Recurring basis, USD $) In Thousands, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013
Level 1 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value			$ 5,006	[1]
Level 1 | Interest rate swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities, fair value
Level 1 | Asset retirement obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities, fair value		[2]		[2]
Level 2 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value				[1]
Level 2 | Interest rate swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities, fair value	715		777
Level 2 | Asset retirement obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities, fair value		[2]		[2]
Level 3 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value				[1]
Level 3 | Interest rate swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities, fair value
Level 3 | Asset retirement obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities, fair value	3,770	[2]	2,357	[2]
Total | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value			5,006	[1]
Total | Interest rate swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities, fair value	715		777
Total | Asset retirement obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities, fair value	$ 3,770	[2]	$ 2,357	[2]

[1] Included in "Cash and cash equivalents" in the consolidated balance sheet as of December 31, 2013. Unrealized gains and losses on money market funds were nominal due to the short-term nature of the investments.

[2] We calculate the fair value of asset retirement obligations by discounting the estimated amount using the current Treasury bill rate adjusted for our credit non-performance. We include current asset retirement obligations of $1.3 million and $1.4 million in "Other current liabilities" in the consolidated balance sheets for September 30, 2014 and December 31, 2013, respectively. We include long-term asset retirement obligations of $2.5 million and $1.0 million in "Other long-term liabilities" in the consolidated balance sheets for September 30, 2014 and December 31, 2013, respectively.