The following table shows our derivative assets and derivative liabilities:



	December 31, 2017				December 31, 2016
(in millions)	Derivative Assets		Derivative Liabilities		Derivative Assets		Derivative Liabilities
Other current
Natural gas contracts	$	5.6	$	9.4	$	31.4	$	0.4
Petroleum products contracts	1.2		—		0.2		0.1
FTRs	4.4		—		5.1		—
Coal contracts	0.6		0.6		1.5		1.4
Total other current	$	11.8	$	10.0	$	38.2	$	1.9

Other long-term
Natural gas contracts	$	0.1	$	1.4	$	2.9	$	—
Coal contracts	0.5		0.2		0.5		0.5
Total other long-term	$	0.6	$	1.6	$	3.4	$	0.5
Total	$	12.4	$	11.6	$	41.6	$	2.4

Our estimated notional sales volumes and realized gains (losses) were as follows for the years ended:



	December 31, 2017				December 31, 2016				December 31, 2015
(in millions)	Volume	Gains (Losses)			Volume	Gains (Losses)			Volume	Gains (losses)
Natural gas contracts	123.1 Dth	$	(8.0	)	151.1 Dth	$	(59.6	)	86.2 Dth	$	(50.5	)
Petroleum products contracts	18.0 gallons	(1.3		)	14.7 gallons	(3.2		)	7.8 gallons	(1.9		)
FTRs	36.2 MWh	14.0			33.7 MWh	13.3			27.3 MWh	6.7
Total		$	4.7			$	(49.5	)		$	(45.7	)

The following table shows derivative assets and derivative liabilities if derivative instruments by counterparty were presented net on our balance sheets:



	December 31, 2017							December 31, 2016
(in millions)	Derivative Assets			Derivative Liabilities				Derivative Assets				Derivative Liabilities
Gross amount recognized on the balance sheet	$	12.4		$	11.6			$	41.6			$	2.4
Gross amount not offset on the balance sheet	(4.9		)	(9.0		)	⁽¹⁾	(4.9		)	⁽²⁾	(0.5		)
Net amount	$	7.5		$	2.6			$	36.7			$	1.9


⁽¹⁾	Includes cash collateral posted of $4.1 million.


⁽²⁾	Includes cash collateral received of $4.4 million.

At December 31, 2017 and 2016, we had posted cash collateral of $16.2 million and $16.4 million, respectively, in our margin accounts. At December 31, 2016, we had also received cash collateral of $4.4 million in our margin accounts. Certain of our derivative and non-derivative commodity instruments contain provisions that could require "adequate assurance" in the event of a material change in our creditworthiness, or the posting of additional collateral for instruments in net liability positions, if triggered by a decrease in credit ratings. The aggregate fair value of all derivative instruments with specific credit risk-related contingent features that were in a net liability position at December 31, 2017 and 2016 was $3.7 million and $0.2 million, respectively. At December 31, 2017 and 2016, we had not posted any cash collateral related to the credit risk-related contingent features of these commodity instruments. If all of the credit risk-related contingent features contained in derivative instruments in a net liability position had been triggered at December 31, 2017, we would have been required to post collateral of $2.7 million. At December 31, 2016, we would not have been required to post any collateral.

During 2015, we settled several forward interest rate swap agreements entered into to mitigate interest rate risk associated with the issuance of $1.2 billion of long-term debt related to the acquisition of Integrys. As these agreements qualified for cash flow hedge accounting treatment, the proceeds of $19.0 million received upon settlement were deferred in accumulated other comprehensive income and are being amortized as a decrease to interest expense over the periods in which the interest costs are recognized in earnings.

For the years ended December 31, 2017, 2016, and 2015, we reclassified $2.2 million, $2.2 million, and $1.2 million, respectively, of forward interest rate swap agreement settlements deferred in accumulated other comprehensive income as a reduction to interest expense. We estimate that during the next twelve months, $2.2 million will be reclassified from accumulated other comprehensive income as a reduction to interest expense.