Derivative Financial Instruments (Tables)

12 Months Ended

Dec. 31, 2017

Derivative Instruments and Hedging Activities Disclosure [Abstract]

Schedule of the gross fair values of derivatives on balance sheet

The gross fair values of derivatives in our consolidated balance sheets are shown below. All balance sheet amounts from derivatives arise from natural gas derivatives that are not designated as hedging instruments. For additional information on derivative fair values, see Note 8—Fair Value Measurements.

	December 31,
	2017			2016
	(in millions)
Derivative Assets
Unrealized gains in other current assets	$	—		$	7.9
Unrealized gains in other assets	—			1.1
Total derivative assets	—			9.0
Derivative Liabilities
Unrealized losses in other current liabilities	(3.2		)	—
Unrealized losses in other liabilities	—			(1.6		)
Total derivative liabilities	(3.2		)	(1.6		)
Net derivative (liabilities) assets	$	(3.2	)	$	7.4

Schedule of effects of derivatives in consolidated statements of operations

The effect of derivatives in our consolidated statements of operations is shown below. All amounts arise from natural gas derivatives that are not designated as hedging instruments and are recorded in cost of goods sold.

	Year ended December 31,
	2017			2016			2015
	(in millions)
Unrealized net mark-to-market (losses) gains	$	(10.6	)	$	35.3		$	(23.1	)
Realized net losses	(3.0		)	(16.7		)	(10.6		)
Net derivative (losses) gains	$	(13.6	)	$	18.6		$	(33.7	)

Schedule of offsetting of derivative assets and liabilities

The following table presents amounts relevant to offsetting of our derivative assets and liabilities as of December 31, 2017 and 2016:

				Gross amounts not offset in  consolidated balance sheets
	Gross and net amounts presented in consolidated balance sheets (1)			Financial instruments			Cash collateral received (pledged)		Net amount
	(in millions)
December 31, 2017
Total derivative assets	$	—		$	—		$	—	$	—
Total derivative liabilities	(3.2		)	—			—		(3.2		)
Net derivative liabilities	$	(3.2	)	$	—		$	—	$	(3.2	)
December 31, 2016
Total derivative assets	$	9.0		$	1.6		$	—	$	7.4
Total derivative liabilities	(1.6		)	(1.6		)	—		—
Net derivative assets	$	7.4		$	—		$	—	$	7.4

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⁽¹⁾  We report the fair values of our derivative assets and liabilities on a gross basis in our consolidated balance sheets. As a result, the gross amounts recognized and net amounts presented herein are the same.