Retirement Plans, Projected Benefit Obligation in Excess of Plan Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Pension Plan [Member]
Projected benefit obligation in excess of plan assets [Abstract]
Projected benefit obligation at end of year	$ 442,861	$ 381,483
Fair value of plan assets at end of year	358,197	303,151
SERP [Member]
Projected benefit obligation in excess of plan assets [Abstract]
Projected benefit obligation at end of year	1,225	1,246
Fair value of plan assets at end of year	$ 0	$ 0