Retirement Plans, Summary of Change in Plan Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Pension Plan [Member]
Change in plan assets [Roll Forward]
Fair value of plan assets at beginning of year	$ 257,517	$ 243,588
Actual return on plan assets	45,196	20,723
Employer contribution	0	0
Gross benefits paid	(7,718)	(6,794)
Fair value of plan assets at end of year	294,995	257,517
SERP [Member]
Change in plan assets [Roll Forward]
Fair value of plan assets at beginning of year	0	0
Actual return on plan assets	0	0
Employer contribution	145	146
Gross benefits paid	(145)	(146)
Fair value of plan assets at end of year	0	0
Other Postretirement Benefits [Member]
Change in plan assets [Roll Forward]
Fair value of plan assets at beginning of year	0	0
Actual return on plan assets	0	0
Employer contribution	75	47
Gross benefits paid	(75)	(47)
Fair value of plan assets at end of year	$ 0	$ 0