Retirement Plans, Projected Benefit Obligation in Excess of Plan Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Pension Plan [Member]
Projected benefit obligation in excess of plan assets [Abstract]
Projected benefit obligation at end of year	$ 354,994	$ 337,176
Fair value of plan assets at end of year	294,995	257,517
SERP [Member]
Projected benefit obligation in excess of plan assets [Abstract]
Projected benefit obligation at end of year	1,412	1,457
Fair value of plan assets at end of year	$ 0	$ 0