Retirement Plans and Other Postretirement Benefits (Tables)

3 Months Ended

Mar. 31, 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   March 31,						Three Months Ended   March 31,
	2016			2015			2016			2015
Service cost — benefits earned during the period	$	14,266		$	15,824		$	3,937		$	4,346
Interest cost on benefit obligation	32,945			31,189			7,341			7,184
Expected return on plan assets	(43,792		)	(45,149		)	(9,122		)	(9,147		)
Amortization of:
Prior service cost	132			149			(9,471		)	(9,492		)
Net actuarial loss	9,731			7,761			946			1,561
Net periodic benefit cost	$	13,282		$	9,774		$	(6,369	)	$	(5,548	)
Portion of cost charged to expense	$	6,519		$	5,987		$	(3,126	)	$	(1,788	)