Fair Value (Tables)

3 Months Ended

Mar. 31, 2018

Fair Value

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

	Fair Value Measurements at Reporting Date Using
		March 31, 2018		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	$	141,117	$	141,117	$	—	$	—
International equity trust		88,148		—		88,148		—
Corporate bonds and debt		62,793		—		60,224		2,569
US Treasury securities		38,707		38,707		—		—
Mortgage backed securities		48,125		—		48,125		—
Domestic mutual funds		47,908		47,908		—		—
Municipal bonds		297		—		297		—
Federal agency securities		4,243		—		4,243		—
Non-US Gov't bonds & private placements		572		—		572		—
Other		8,740		8,740		—		—
Long-term investments:
International equity trust		19,923		—		19,923		—
Corporate bonds and debt		15,855		—		14,617		1,238
US Treasury securities		7,147		7,147		—		—
Mortgage backed securities		6,645		—		6,645		—
Domestic mutual funds		90,134		90,134		—		—
Federal agency securities		384		—		384		—
Other		4,172		4,172		—		—
Natural gas swaps		6,903		—		6,903		—

	Fair Value Measurements at Reporting Date Using
		December 31, 2017		Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)
		(dollars in thousands)
Nuclear decommissioning trust funds:
Domestic equity	$	142,419	$	142,419	$	—
International equity trust		88,820		—		88,820
Corporate bonds and debt		66,317		—		66,317
US Treasury securities		38,791		38,791		—
Mortgage backed securities		49,379		—		49,379
Domestic mutual funds		47,833		47,833		—
Municipal bonds		92		—		92
Federal agency securities		3,725		—		3,725
Other		7,679		7,679		—
Long-term investments:
International equity trust		20,071		—		20,071
Corporate bonds and debt		16,215		—		16,215
US Treasury securities		6,670		6,670		—
Mortgage backed securities		7,267		—		7,267
Domestic mutual funds		87,011		87,011		—
Federal agency securities		259		—		259
Other		3,129		3,129		—
Natural gas swaps		6,328		—		6,328

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

	Three Months Ended March 31, 2018
		Corporate bonds and debt
		(dollars in thousands)
Balance at December 31, 2017	$	—
Transfers to Level 3		3,807
Total gains or losses (realized/unrealized):
Changes in net assets		—
Balance at March 31, 2018	$	3,807

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 March 31, 2018 and December 31, 2017 were as follows (in thousands):

		2018				2017
		Carrying Value		Fair Value		Carrying Value		Fair Value
Long-term debt	$	8,212,644	$	8,838,667	$	8,232,703	$	9,155,942