Retirement Plans and Other Postretirement Benefits (Tables)

6 Months Ended

Jun. 30, 2017

Compensation and Retirement Disclosure [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, billed to electric plant participants or charged to the regulatory asset or liability) (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,
	2017			2016			2017			2016			2017			2016			2017			2016
Service cost — benefits earned during the period	$	13,669		$	12,630		$	27,429		$	26,896		$	4,201		$	3,560		$	8,559		$	7,497
Interest cost on benefit obligation	32,177			32,878			64,878			65,823			7,415			7,519			14,980			14,860
Expected return on plan assets	(43,425		)	(43,161		)	(87,135		)	(86,953		)	(13,350		)	(9,125		)	(26,701		)	(18,247		)
Amortization of:
Prior service cost (credit)	20			132			41			263			(9,461		)	(9,471		)	(18,921		)	(18,942		)
Net actuarial loss	11,460			10,627			23,950			20,358			1,104			1,349			2,559			2,295
Net periodic benefit cost	$	13,901		$	13,106		$	29,163		$	26,387		$	(10,091	)	$	(6,168	)	$	(19,524	)	$	(12,537	)
Portion of cost charged to expense	$	6,894		$	6,433		$	14,461		$	12,951		$	(5,004	)	$	(3,027	)	$	(9,682	)	$	(6,153	)