Note 6 - Earnings Per Share - Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended		18 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023
Net income	$ 93,220	$ 88,978	$ 189,227	$ 164,006	$ 189,200
Basic weighted-average number of common shares outstanding (in shares)	72,536,301	74,958,913	72,534,779	75,144,414
Dilutive effect of weighted-average outstanding common share equivalents RSUs (in shares)	217,445	311,227	291,522	349,102
Diluted weighted-average number of common shares outstanding (in shares)	72,753,746	75,270,140	72,826,301	75,493,516
Average restricted stock units with anti-dilutive effect (in shares)	5,527	108,211	3,165	65,629
Basic (in dollars per share)	$ 1.29	$ 1.19	$ 2.61	$ 2.18
Diluted (in dollars per share)	$ 1.28	$ 1.18	$ 2.60	$ 2.17