Retirement Plans and Other Postretirement Benefits (Tables)

6 Months Ended

Jun. 30, 2016

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 or amortized to the regulatory asset) (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,
	2016			2015			2016			2015			2016			2015			2016			2015
Service cost — benefits earned during the period	$	12,630		$	13,990		$	26,896		$	29,814		$	3,560		$	4,068		$	7,497		$	8,413
Interest cost on benefit obligation	32,878			30,802			65,823			61,992			7,519			6,867			14,860			14,051
Expected return on plan assets	(43,161		)	(44,467		)	(86,953		)	(89,616		)	(9,125		)	(9,281		)	(18,247		)	(18,428		)
Amortization of:
Prior service cost	132			149			263			297			(9,471		)	(9,492		)	(18,942		)	(18,984		)
Net actuarial loss	10,627			7,767			20,358			15,528			1,349			880			2,295			2,441
Net periodic benefit cost	$	13,106		$	8,241		$	26,387		$	18,015		$	(6,168	)	$	(6,958	)	$	(12,537	)	$	(12,507	)
Portion of cost charged to expense	$	6,433		$	5,232		$	12,951		$	11,219		$	(3,027	)	$	(2,482	)	$	(6,153	)	$	(4,271	)