Fair Value (Tables)

3 Months Ended

Mar. 31, 2016

Fair Value

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

	Fair Value Measurements at Reporting Date Using
		March 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	$	152,412	$	152,412	$	—	$	—
International equity trust		69,040		—		69,040		—
Corporate bonds		46,223		—		46,223		—
US Treasury and government agency securities		80,644		80,644		—		—
Agency mortgage and asset backed securities		16,463		—		16,463		—
Municipal bonds		380		—		380		—
Other		4,279		4,279		—		—
Long-term investments:
International equity trust		12,904		—		12,904		—
Corporate bonds		9,516		—		9,516		—
US Treasury and government agency securities		14,692		14,692		—		—
Agency mortgage and asset backed securities		1,482		—		1,482		—
Mutual funds		51,721		51,721		—		—
Other		155		155		—		—
Interest rate options		95		—		—		95
Natural gas swaps		25,919		—		25,919		—

	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
Natural gas swaps		24,995		—		24,995		—

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

		Three Months Ended March 31, 2016
		Interest rate options
		(dollars in thousands)
Assets (Liabilities):
Balance at December 31, 2015	$	1,010
Total gains or losses (realized/unrealized):
Included in earnings (or changes in net assets)		(915	)
Balance at March 31, 2016	$	95

		Three Months Ended March 31, 2015
		Interest rate options
		(dollars in thousands)
Assets (Liabilities):
Balance at December 31, 2014	$	4,371
Total gains or losses (realized/unrealized):
Included in earnings (or changes in net assets)		(1,669	)
Balance at March 31, 2015	$	2,702

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

		2016				2015
		Carrying Value		Fair Value		Carrying Value		Fair Value
Long-term debt	$	7,686,691	$	8,991,014	$	7,575,027	$	8,445,630