Derivative instruments: (Tables)

12 Months Ended

Dec. 31, 2015

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, 2015 that is expected to settle or mature each year:

Year	Natural Gas Swaps (MMBTUs)
	(in millions)
2016	23.1
2017	15.6
2018	10.0
2019	6.0
Total	54.7

Schedule of remaining notional amount of forecasted debt issuances hedged in each year with LIBOR swaptions

    The following table reflects the remaining notional amount of forecasted debt issuances we have hedged in each year with LIBOR swaptions as of December 31, 2015.

Year		LIBOR Swaption Notional Dollar Amount
		(in thousands)
2016	$	310,533
2017		80,169
Total	$	390,702

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

	Balance Sheet Location		Fair Value
			2015		2014
			(dollars in thousands)
Not designated as hedge:
Assets
Interest rate options	Other deferred charges	$	1,010	$	4,371
Liabilities
Natural gas swaps	Other current liabilities	$	22,848	$	18,914

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

	Consolidated Statement of Revenues and Expenses Location		2015			2014			2013
			(dollars in thousands)
Not designated as hedge:
Natural Gas Swaps	Fuel	$	206		$	1,881		$	739
Natural Gas Swaps	Fuel		(20,102	)		(1,033	)		(4,051	)
		$	(19,896	)	$	848		$	(3,312	)

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

	Consolidated Balance Sheet Location
			2015			2014
Not designated as hedges:
Natural Gas Swaps	Regulatory asset	$	(22,848	)	$	(18,914	)
Interest Rate Options	Regulatory asset		(25,915	)		(49,232	)
Total not designated as hedges		$	(48,763	)	$	(68,146	)

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, 2015
Assets:
Natural gas swaps	$	(22,848	)	$	–		$	–	$	(22,848	)
Interest rate options	$	26,925		$	(25,915	)	$	–	$	1,010
December 31, 2014
Assets:
Natural gas swaps	$	(18,914	)	$	–		$	–	$	(18,914	)
Interest rate options	$	53,603		$	(49,232	)	$	–	$	4,371