Pension and Postretirement Benefits (Tables)

3 Months Ended

Mar. 31, 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 March 31,
	Pension						Postretirement Benefits
	2017			2016			2017			2016
	(Dollars in Thousands)
Service cost	$	6,572		$	5,507		$	24		$	34
Interest cost		9,008			8,871			185			238
Expected return on assets		(9,936	)		(9,434	)		—			—
Amortization of:
Prior service cost (credit)		78			88			(435	)		(462	)
Actuarial loss (gain)		3,524			2,831			(72	)		(99	)
Settlement charge		—			59			—			—
Special termination benefit		—			126			—			—
Net periodic benefit cost (credit)	$	9,246		$	8,048		$	(298	)	$	(289	)