Retirement Plans (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Company's pension costs
Pension payment timing related charges	$ 1,200	$ 6,300
Retirement Plans
Company's pension costs
Service cost-benefits earned during the period	3		$ 218	$ 9	$ 654
Interest cost on benefit obligation	5,721		5,990	15,945	17,971
Expected return on plan assets	(8,218)		(6,580)	(23,148)	(19,741)
Amortization of prior service cost	42		159	126	476
Amortization of actuarial loss	1,271		2,081	3,814	6,242
Pension payment timing related charges	1,198		0	7,498	0
Expense for company-sponsored retirement plans	$ 17		$ 1,868	$ 4,244	$ 5,602