Fair Value Measurements

12 Months Ended

Dec. 31, 2011

Fair Value Measurements

Fair Value Measurements

8.     Fair Value Measurements

Assets and Liabilities Measured at Fair Value on a Recurring Basis

        The following table presents assets and liabilities included in our Consolidated Balance Sheets that are recognized at fair value on a recurring basis, and indicates the fair value hierarchy utilized to determine such fair value as of December 31, 2011 and 2010.

	Balances as of December 31, 2011
	Total			Quoted Market Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)			Significant Unobservable Inputs (Level 3)
	(in millions)
Cash and cash equivalents	$	179.8		$	179.8	$	—		$	—
Total assets at fair value	$	179.8		$	179.8	$	—		$	—
Unrealized losses on natural gas derivatives	$	(12.0	)	$	—	$	(12.0	)	$	—
Total liabilities at fair value	$	(12.0	)	$	—	$	(12.0	)	$	—

	Balances as of December 31, 2010
	Total			Quoted Market Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)			Significant Unobservable Inputs (Level 3)
	(in millions)
Cash and cash equivalents	$	124.8		$	124.8	$	—		$	—
Unrealized gains on natural gas derivatives		0.7			—		0.7			—
Total assets at fair value	$	125.5		$	124.8	$	0.7		$	—
Unrealized losses on natural gas derivatives	$	(0.8	)	$	—	$	(0.8	)	$	—
Total liabilities at fair value	$	(0.8	)	$	—	$	(0.8	)	$	—

        Following is a summary of the valuation techniques for assets and liabilities recorded in our Consolidated Balance Sheets at fair value on a recurring basis:

Cash and Cash Equivalents

        As of December 31, 2011 and 2010, our cash and cash equivalents consisted primarily of U.S. Treasury Bills with original maturities of three months or less and money market mutual funds that invest in U.S. government obligations.

Natural Gas Derivatives

        The derivative instruments that we currently use are primarily natural gas swap contracts. These contracts settle using NYMEX futures prices, which represent fair value at any given time. The contracts are traded in months forward and settlements are scheduled to coincide with anticipated gas purchases during those future periods. Quoted market prices from NYMEX are used to determine the fair value of these instruments. See Note 7—Derivative Financial Instruments for additional information.