Retirement Plans and Other Postretirement Benefits (Tables)

6 Months Ended

Jun. 30, 2018

Retirement Benefits [Abstract]

Schedule of net periodic benefit costs and the portion of these costs charged to expense (including administrative costs and excluding amounts capitalized as overhead construction, billed to electric plant participants or charged or amortized to the regulatory asset)

The following table provides details of the plans’ net periodic benefit costs and the portion of these costs charged to expense (including administrative costs and excluding amounts capitalized as overhead construction or billed to electric plant participants) (dollars in thousands):

	Pension Benefits												Other Benefits
	Three Months Ended   June 30,						Six Months Ended   June 30,						Three Months Ended   June 30,						Six Months Ended   June 30,
	2018			2017			2018			2017			2018			2017			2018			2017
Service cost — benefits earned during the period	$	14,121		$	13,669		$	28,334		$	27,429		$	5,445		$	4,201		$	10,550		$	8,559
Non-service costs (credits):
Interest cost on benefit obligation	31,338			32,177			62,345			64,878			6,973			7,415			14,074			14,980
Expected return on plan assets	(45,759		)	(43,425		)	(91,426		)	(87,135		)	(10,520		)	(13,350		)	(21,041		)	(26,701		)
Amortization of:
Prior service cost (credit)	—			20			—			41			(9,461		)	(9,461		)	(18,921		)	(18,921		)
Net actuarial loss	8,259			11,460			16,041			23,950			—			1,104			—			2,559
Net periodic benefit cost (credit)	$	7,959		$	13,901		$	15,294		$	29,163		$	(7,563	)	$	(10,091	)	$	(15,338	)	$	(19,524	)
Portion of cost (credit) charged to expense	$	2,769		$	6,894		$	5,011		$	14,461		$	(5,119	)	$	(5,004	)	$	(10,724	)	$	(9,682	)