Note 9 - Employee Benefit Plans - Change in Benefit Obligation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Benefit obligation at beginning of year	$ 172,909	$ 144,638
Service cost	5,334	4,401	$ 5,009
Interest cost	5,618	6,482	5,508
Change due to mortality change	(1,521)	(2,130)
Change due to discount rate change	17,040	25,806
Actuarial return (loss)	121	(395)
Benefits paid	(6,547)	(5,893)
Benefit obligation at end of year	192,954	172,909	144,638
Fair value of plan assets at beginning of year	116,067	96,329
Actual return on plan assets	14,801	21,536
Employer contributions	23,731	4,095
Benefits paid	(6,547)	(5,893)
Fair value of plan assets at end of year	148,052	116,067	$ 96,329
Underfunded status at end of year	$ (44,902)	$ (56,842)