9.REGULATORY ASSETS AND LIABILITIES

The table below presents a summary of regulatory assets, net of amortization, and liabilities for the periods indicated:



		December 31, 2018
	Remaining Recovery Period	Current			Noncurrent			Total
		(Thousands of dollars)
Under-recovered purchased-gas costs	1 year	$	25,083		$	—		$	25,083
Pension and other postemployment benefit costs	See Note 12	23,384			421,726			445,110
Reacquired debt costs	9 years	812			6,487			7,299
MGP remediation costs	15 years	—			7,724			7,724
Ad-valorem tax	1 year	1,070			—			1,070
Other	1 to 20 years	4,071			1,542			5,613
Total regulatory assets, net of amortization		54,420			437,479			491,899
Federal income tax rate changes (a)	See Note 13	(30,934		)	(520,866		)	(551,800		)
Over-recovered purchased-gas costs	1 year	(13,668		)	—			(13,668		)
Weather normalization	1 year	(3,792		)	—			(3,792		)
Total regulatory liabilities		(48,394		)	(520,866		)	(569,260		)
Net regulatory assets and liabilities		$	6,026		$	(83,387	)	$	(77,361	)

(a) See Note 13 for additional information regarding our federal income tax rate changes regulatory liabilities.



		December 31, 2017
	Remaining Recovery Period	Current			Noncurrent			Total
		(Thousands of dollars)
Under-recovered purchased-gas costs	1 year	$	41,238		$	—		$	41,238
Pension and other postemployment benefit costs	See Note 12	25,156			387,582			412,738
Weather normalization	1 year	17,461			—			17,461
Reacquired debt costs	10 years	812			7,298			8,110
MGP remediation costs	15 years	—			6,104			6,104
Other	1 to 21 years	3,513			4,205			7,718
Total regulatory assets, net of amortization		88,180			405,189			493,369
Federal income tax rate changes (a)	See Note 13	—			(519,421		)	(519,421		)
Over-recovered purchased-gas costs	1 year	(9,434		)	—			(9,434		)
Ad-valorem tax	1 year	(4		)	—			(4		)
Total regulatory liabilities		(9,438		)	(519,421		)	(528,859		)
Net regulatory assets and liabilities		$	78,742		$	(114,232	)	$	(35,490	)

(a) See Note 13 for additional information regarding our federal income tax rate changes regulatory liabilities.

Regulatory assets on our Consolidated Balance Sheets, as authorized by the various regulatory authorities, are probable of recovery. Base rates are designed to provide a recovery of cost during the period rates are in effect but do not generally provide for a return on investment for amounts we have deferred as regulatory assets. All of our regulatory assets recoverable through base rates are subject to review by the respective regulatory authorities during future rate proceedings. We are not aware of any evidence that these costs will not be recoverable through either rate riders or base rates, and we believe that we will be able to recover such costs, consistent with our historical recoveries.

Purchased-gas costs represent the natural gas costs that have been over- or under-recovered from customers through the purchased-gas cost adjustment mechanisms, and includes natural gas utilized in our operations and premiums paid and any cash settlements received from our purchased natural gas call options.

We amortize reacquired debt costs in accordance with the accounting guidelines prescribed by the OCC and KCC.

Weather normalization represents revenue over- or under-recovered through the WNA rider in Kansas. This amount is deferred as a regulatory asset or liability for a 12-month period. Kansas Gas Service then applies an adjustment to the customers’ bills for 12 months to refund the over-collected revenue or bill the under-collected revenue.

Ad-valorem tax represents an increase or decrease in Kansas Gas Service’s taxes above or below the amount approved in a rate case. This amount is deferred as a regulatory asset or liability for a 12-month period. Kansas Gas Service then applies an adjustment to the customers’ bills for 12 months to refund the over-collected revenue or bill the under-collected revenue.

Recovery through rates resulted in amortization of regulatory assets of approximately $1.7 million, $1.0 million and $3.8 million for the years ended December 31, 2018, 2017 and 2016, respectively.

In 2017, we recorded a regulatory asset of approximately $5.9 million for estimated costs expected to be incurred at, and nearby, our 12 former MGP sites in Kansas which we own or retain responsibility for certain environmental conditions.