Note 6 - Earnings Per Common Share - Basic and Diluted Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net income	$ 65,074	$ 7,855	$ 9,141	$ 8,214	$ 8,303	$ 8,437	$ 8,531	$ 8,193	$ 90,284	$ 33,464	$ 35,745
Denominator:
Basic earnings per common share - weighted-average shares (in shares)									54,492	54,177	55,796
Effect of dilutive stock options (in shares)									358	284	395
Diluted earnings per common share - weighted-average shares and assumed conversions (in shares)									54,850	54,461	56,191
Basic earnings per common share (in dollars per share)	$ 1.19	$ 0.14	$ 0.17	$ 0.15	$ 0.15	$ 0.16	$ 0.16	$ 0.15	$ 1.66	$ 0.62	$ 0.64
Diluted earnings per common share (in dollars per share)	$ 1.18	$ 0.14	$ 0.17	$ 0.15	$ 0.15	$ 0.15	$ 0.16	$ 0.15	$ 1.65	$ 0.61	$ 0.64