Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2013

Fair Value Measurements

Schedule of assets and liabilities that are recognized at fair value on a recurring basis

	Balances as of December 31, 2013
	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	$	86.9	$	86.9	$	—	$	—
Unrealized gains on natural gas derivatives		7.8		—		7.8		—
Total assets at fair value	$	94.7	$	86.9	$	7.8	$	—

	Balances as of December 31, 2012
	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	$	149.4		$	149.4	$	—		$	—
Unrealized gains on natural gas derivatives		0.2			—		0.2			—
Total assets at fair value	$	149.6		$	149.4	$	0.2		$	—
Unrealized losses on natural gas derivatives	$	(1.0	)	$	—	$	(1.0	)	$	—
Total liabilities at fair value	$	(1.0	)	$	—	$	(1.0	)	$	—