Pension and Postretirement Benefits (Tables)

6 Months Ended

Jun. 30, 2018

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
	2018			2017			2018			2017
	(Dollars in Thousands)
Service cost	$	7,684		$	6,548		$	16		$	22
Interest cost		8,252			8,673			134			198
Expected return on assets		(12,403	)		(10,071	)		—			—
Amortization of:
Prior service cost (credit)		26			113			(426	)		(471	)
Actuarial loss (gain)		3,117			3,551			(48	)		(99	)
Net periodic benefit cost (credit)	$	6,676		$	8,814		$	(324	)	$	(350	)

	Six-Months Ended June 30,
	Pension						Postretirement Benefits
	2018			2017			2018			2017
	(Dollars in Thousands)
Service cost	$	15,832		$	13,402		$	38		$	40
Interest cost		16,613			18,030			259			365
Expected return on assets		(23,032	)		(20,613	)		—			—
Amortization of:
Prior service cost (credit)		52			155			(1,037	)		(871	)
Actuarial loss (gain)		6,413			7,087			(105	)		(182	)
Net periodic benefit cost (credit)	$	15,878		$	18,061		$	(845	)	$	(648	)