Derivative instruments: (Tables)

12 Months Ended

Dec. 31, 2016

Derivative instruments:

Schedule of volume activity of natural gas derivatives that is expected to settle or mature each year

    The following table reflects the volume activity of our natural gas derivatives as of December 31, 2016 that is expected to settle or mature each year:

Year	Natural Gas Swaps (MMBTUs)
	(in millions)
2017	23.2
2018	20.4
2019	13.5
2020	9.0
2021	2.5
Total	68.6

Schedule of fair value of derivative instruments and their effect on consolidated balance sheets

	Balance Sheet Location		Fair Value
			2016		2015
			(dollars in thousands)
Not designated as hedges:
Assets
Interest rate options	Other deferred charges	$	–	$	1,010
Natural gas swaps	Other current assets	$	13,833		–
Natural gas swaps	Other deferred charges	$	3,289		–
Liabilities
Natural gas swaps	Other current liabilities	$	54	$	22,848
Natural gas swaps	Other deferred credits	$	1,977	$	–

Schedule of the gross realized gains and (losses) on derivative instruments recognized in margin

	Consolidated Statement of Revenues and Expenses Location		2016			2015			2014
			(dollars in thousands)
Not designated as hedges:
Natural Gas Swaps	Fuel	$	2,445		$	206		$	1,881
Natural Gas Swaps	Fuel		(19,697	)		(20,102	)		(1,033	)
		$	(17,252	)	$	(19,896	)	$	848

Schedule of unrealized gains and (losses) on derivative instruments deferred on the balance sheet

	Consolidated Balance Sheet Location
			2016			2015
Not designated as hedges:
Natural Gas Swaps	Regulatory asset	$	(62	)	$	(22,848	)
Natural Gas Swaps	Regulatory liability		15,152
Interest Rate Options	Regulatory asset		(5,788	)		(25,915	)
Total not designated as hedges		$	9,302		$	(48,763	)

Schedule of gross amounts of derivatives and their related offset amounts as permitted by their respective master netting agreements and obligations to return cash collateral

		(dollars in thousands)
		Gross Amounts of Recognized Assets (Liabilities)			Gross Amounts offset on the Balance Sheet			Cash Collateral		Net Amounts of Assets (Liabilities) Presented on the Balance Sheet
December 31, 2016
Natural gas swaps	$	15,090		$	–		$	–	$	15,090
Natural gas swaps	$	(15,090	)	$	–		$	–	$	(15,090	)
Interest rate options	$	5,788		$	(5,788	)	$	–	$	–
December 31, 2015
Assets:
Natural gas swaps	$	(22,848	)	$	–		$	–	$	(22,848	)
Interest rate options	$	26,925		$	(25,915	)	$	–	$	1,010