Note 8 - Employee Benefit Plans - Change in Benefit Obligation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Benefit obligation at beginning of year	$ 144,638	$ 147,023
Service cost	4,401	5,009	$ 4,786
Interest cost	6,482	5,508	5,356
Amendments		0
Change due to mortality change	(2,130)	2,713
Change due to discount rate change	25,806	(13,716)
Actuarial loss	(395)	3,487
Benefits paid	(5,893)	(5,386)
Benefit obligation at end of year	172,909	144,638	147,023
Fair value of plan assets at beginning of year	96,329	99,922
Actual (loss) return on plan assets	21,536	(8,772)
Employer contributions	4,095	10,565
Benefits paid	(5,893)	(5,386)
Fair value of plan assets at end of year	116,067	96,329	$ 99,922
Underfunded status at end of year	$ (56,842)	$ (48,309)