Pension and Postretirement Benefits (Tables)

6 Months Ended

Jun. 30, 2017

Compensation And Retirement Disclosure [Abstract]

Schedule of Components of Net Periodic Benefit Cost (Credit)

The estimated components of the recorded net periodic benefit cost (credit) for pension and postretirement benefits are as follows:

	Three Months Ended June 30,
	Pension						Postretirement Benefits
	2017			2016			2017			2016
	(Dollars in Thousands)
Service cost	$	6,548		$	5,776		$	22		$	24
Interest cost		8,673			9,331			198			259
Expected return on assets		(10,071	)		(9,822	)		—			—
Amortization of:
Prior service cost (credit)		113			91			(471	)		(609	)
Actuarial loss (gain)		3,551			3,146			(99	)		(139	)
Special termination benefit		—			638			—			(8	)
Net periodic benefit cost (credit)	$	8,814		$	9,160		$	(350	)	$	(473	)

	Six Months Ended June 30,
	Pension						Postretirement Benefits
	2017			2016			2017			2016
	(Dollars in Thousands)
Service cost	$	13,402		$	11,082		$	40		$	43
Interest cost		18,030			17,938			365			432
Expected return on assets		(20,613	)		(18,848	)		—			—
Amortization of:
Prior service cost (credit)		155			175			(871	)		(981	)
Actuarial loss (gain)		7,087			6,036			(182	)		(249	)
Settlement charge		—			59			—			—
Special termination benefit		—			764			—			(8	)
Net periodic benefit cost (credit)	$	18,061		$	17,206		$	(648	)	$	(763	)