Retirement Plans, Projected Benefit Obligation in Excess of Plan Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Pension Plan [Member]
Projected benefit obligation in excess of plan assets [Abstract]
Projected benefit obligation at end of year	$ 381,483	$ 354,994
Fair value of plan assets at end of year	303,151	294,995
SERP [Member]
Projected benefit obligation in excess of plan assets [Abstract]
Projected benefit obligation at end of year	1,246	1,412
Fair value of plan assets at end of year	$ 0	$ 0