Fair Value: (Tables)

12 Months Ended

Dec. 31, 2018

Fair Value:

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

	Fair Value Measurements at Reporting Date Using
			Quoted Prices in		Significant Other		Significant
			Active Markets for		Observable		Unobservable
			Identical Assets		Inputs		Inputs
	December 31, 2018		(Level 1)		(Level 2)		(Level 3)
	(dollars in thousands)
Nuclear decommissioning trust funds:
Domestic equity	$	136,196	$	136,196	$	—	$	—
International equity trust	$	76,852		—		76,852		—
Corporate bonds and debt	$	51,356		—		48,853		2,503
US Treasury securities	$	47,712		47,712		—		—
Mortgage backed securities	$	56,004		—		56,004		—
Domestic mutual funds	$	43,359		43,359				—
Municipal bonds	$	278		—		278		—
Federal agency securities	$	6,066		—		6,066		—
Other	$	2,995		2,031		964		—
Long-term investments:
International equity trust	$	17,382		—		17,382		—
Corporate bonds and debt	$	12,571		—		11,366		1,205
US Treasury securities	$	12,062		12,062		—		—
Mortgage backed securities	$	11,517		—		11,517		—
Domestic mutual funds	$	94,494		94,494		—		—
Federal agency securities	$	941		—		941		—
Treasury STRIPS	$	14,113		—		14,113		—
Other	$	1,045		1,045		—		—
Natural gas swaps	$	13,154		—		13,154		—

	Fair Value Measurements at Reporting Date Using
			Quoted Prices in		Significant Other		Significant
			Active Markets for		Observable		Unobservable
			Identical Assets		Inputs		Inputs
	December 31, 2017		(Level 1)		(Level 2)		(Level 3)
	(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

	Year Ended
	December 31, 2018
	(dollars in thousands)
Balance at December 31, 2017	$	—
Transfers to Level 3		4,997
Total gains or losses (realized/unrealized):
Changes in net assets		(1,289)
Balance at December 31, 2018	$	3,708

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

	2018				2017
	Carrying		Fair		Carrying		Fair
	Value		Value		Value		Value
Long-term debt	$	9,347,307	$	9,837,254	$	8,232,703	$	9,155,942