Retirement Plans, Schedule of Change in Benefit Obligation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plan [Member]
Change in benefit obligation [Roll Forward]
Benefit obligation at beginning of year	$ 337,176	$ 297,325
Service cost	10,677	13,402	$ 14,683
Interest cost	13,729	14,123	13,302
Actuarial loss (gain)	34,563	19,120
Curtailments	(33,433)	0
Gross benefits paid	(7,718)	(6,794)
Benefit obligation at end of year	354,994	337,176	297,325
Accumulated benefit obligation at end of year	324,904	272,591
SERP [Member]
Change in benefit obligation [Roll Forward]
Benefit obligation at beginning of year	1,457	1,526
Service cost	0	0	0
Interest cost	58	65	64
Actuarial loss (gain)	42	12
Curtailments	0	0
Gross benefits paid	(145)	(146)
Benefit obligation at end of year	1,412	1,457	1,526
Accumulated benefit obligation at end of year	1,412	1,457
Other Postretirement Benefits [Member]
Change in benefit obligation [Roll Forward]
Benefit obligation at beginning of year	675	891
Service cost	0	0	0
Interest cost	27	29	36
Actuarial loss (gain)	52	(198)
Curtailments	0	0
Gross benefits paid	(75)	(47)
Benefit obligation at end of year	679	675	$ 891
Accumulated benefit obligation at end of year	$ 679	$ 675