Retirement Benefits (Tables)

6 Months Ended

Jun. 30, 2018

Retirement Benefits [Abstract]

Components of Net Periodic Benefit Cost

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

	Three Months Ended						Six Months Ended
	June 30, 2018			June 30, 2017			June 30, 2018			June 30, 2017
	(Dollars in thousands)
Service cost	$	3,710		$	3,267		$	7,431		$	6,435
Interest cost	6,296			6,362			12,605			12,632
Expected return on plan assets	(17,122		)	(15,713		)	(34,245		)	(31,426		)
Amortization of prior service cost	34			80			69			160
Amortization of actuarial losses	1,787			1,854			3,573			3,707
Net periodic benefit credit	$	(5,295	)	$	(4,150	)	$	(10,567	)	$	(8,492	)

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

	Three Months Ended						Six Months Ended
	June 30, 2018			June 30, 2017			June 30, 2018			June 30, 2017
	(Dollars in thousands)
Service cost	$	32		$	34		$	63		$	70
Interest cost	162			176			325			352
Amortization of prior service credit	(650		)	(854		)	(1,299		)	(1,707		)
Amortization of actuarial gains	(119		)	(136		)	(238		)	(275		)
Net periodic benefit credit	$	(575	)	$	(780	)	$	(1,149	)	$	(1,560	)