Retirement Benefits (Tables)

6 Months Ended

Jun. 30, 2017

Retirement Benefits [Abstract]

Components of Net Periodic Benefit Cost

The components of the net periodic pension benefit (credit) cost were as follows:

	Three Months Ended						Six Months Ended
	June 30, 2017			June 30, 2016			June 30, 2017			June 30, 2016
	(Dollars in thousands)
Service cost	$	3,267		$	3,321		$	6,435		$	6,634
Interest cost	6,362			6,444			12,632			12,878
Expected return on plan assets	(15,713		)	(14,583		)	(31,426		)	(29,166		)
Amortization of prior service cost	80			151			160			302
Amortization of actuarial losses	1,854			2,084			3,707			4,167
Special termination benefits	—			2,900			—			2,900
Curtailment loss	—			180			—			180
Net periodic benefit (credit) cost	$	(4,150	)	$	497		$	(8,492	)	$	(2,105	)

The components of the net periodic other postretirement benefit credit were as follows:

	Three Months Ended						Six Months Ended
	June 30, 2017			June 30, 2016			June 30, 2017			June 30, 2016
	(Dollars in thousands)
Service cost	$	34		$	69		$	70		$	136
Interest cost	176			253			352			507
Amortization of prior service credit	(854		)	(850		)	(1,707		)	(1,700		)
Amortization of actuarial gains	(136		)	(170		)	(275		)	(288		)
Net periodic benefit credit	$	(780	)	$	(698	)	$	(1,560	)	$	(1,345	)