Fair Value: (Tables)

12 Months Ended

Dec. 31, 2016

Fair Value:

Schedule of assets and liabilities measured at fair value on a recurring basis

	Fair Value Measurements at Reporting Date Using
		December 31, 2016			Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)			Significant Unobservable Inputs (Level 3)
		(dollars in thousands)
Nuclear decommissioning trust funds:
Domestic equity	$	170,408		$	170,408	$	–		$	–
International equity trust		66,861			–		66,861			–
Corporate bonds		60,019			–		60,019			–
US Treasury and government agency securities		65,725			65,725		–			–
Agency mortgage and asset backed securities		17,410			–		17,410			–
Municipal Bonds		943			–		943			–
Other		4,663			4,663		–			–
Long-term investments:
Corporate bonds		11,853			–		11,853			–
US Treasury and government agency securities		12,187			12,187		–			–
Agency mortgage and asset backed securities		1,651			–		1,651			–
International equity trust		15,946			–		15,946			–
Mutual funds		57,932			57,932		–			–
Other		305			305		–			–
Natural gas swaps		(15,090	)		–		(15,090	)		–

	Fair Value Measurements at Reporting Date Using
		December 31, 2015		Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
		(dollars in thousands)
Nuclear decommissioning trust funds:
Domestic equity	$	151,178	$	151,178	$	–	$	–
International equity trust		68,753		–		68,753		–
Corporate bonds		48,450		–		48,450		–
US Treasury and government agency securities		75,173		74,698		475		–
Agency mortgage and asset backed securities		15,503		–		15,503		–
Other		4,772		4,772		–		–
Long-term investments:
Corporate bonds		9,903		–		9,903		–
US Treasury and government agency securities		13,772		13,772		–		–
Agency mortgage and asset backed securities		1,121		–		1,121		–
International equity trust		12,846		–		12,846		–
Mutual funds		48,649		48,649		–		–
Other		479		479		–		–
Interest rate options		1,010		–		–		1,010	(1)
Natural gas swaps		24,995		–		24,995		–

(1) Interest rate options as reflected on the Consolidated Balance Sheet include the fair value of the interest rate options.

Schedule of changes in Level 3 assets measured at fair value on a recurring basis

	Year Ended December 31, 2016
		Interest rate options
		(dollars in thousands)
Assets:
Balance at December 31, 2015	$	1,010
Total gains or losses (realized/unrealized):
Included in earnings (or changes in net assets)		(1,010	)
Balance at December 31, 2016	$	–

	Year Ended December 31, 2015
		Interest rate options
		(dollars in thousands)
Assets:
Balance at December 31, 2014	$	4,371
Total gains or losses (realized/unrealized):
Included in earnings (or changes in net assets)		(3,361	)
Balance at December 31, 2015	$	1,010

Schedule of estimated fair values of long-term debt, including current maturities

    The estimated fair values of our long-term debt, including current maturities at December 31, 2016 and 2015 were as follows (in thousands):

		2016				2015
		Carrying Value		Fair Value		Carrying Value		Fair Value
Long-term debt	$	8,304,523	$	9,043,029	$	7,575,027	$	8,445,630