Fair Value: (Tables)

12 Months Ended

Dec. 31, 2015

Fair Value:

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

	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		–

		Fair Value Measurements at Reporting Date Using
			December 31, 2014		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		$	159,536	$	159,536	$	–	$	–
International equity trust			72,474		–		72,474		–
Corporate bonds			34,446		–		34,446		–
US Treasury and government agency securities			68,854		68,854		–		–
Agency mortgage and asset backed securities			16,148		–		16,148		–
Municipal Bonds			743		–		743		–
Other			13,803		13,803		–		–
Long-term investments:
Corporate bonds			5,445		–		5,445		–
US Treasury and government agency securities			16,619		16,619		–		–
Agency mortgage and asset backed securities			643		–		643		–
International equity trust			11,162		–		11,162		–
Mutual funds			51,741		51,741		–		–
Other			118		118		–		–
Interest rate options			4,371		–		–		4,371	(1)
Natural gas swaps			18,914		–		18,914		–
(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, 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

	Year Ended December 31, 2014
		Interest rate options
		(dollars in thousands)
Assets:
Balance at December 31, 2013	$	63,471
Total gains or losses (realized/unrealized):
Included in earnings (or changes in net assets)		(59,100	)
Balance at December 31, 2014	$	4,371

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, 2015 and 2014 were as follows (in thousands):

		2015				2014
		Carrying Value		Fair Value		Carrying Value		Fair Value
Long-term debt	$	7,575,027	$	8,445,630	$	7,256,995	$	8,460,685