Note 4 - Earnings Per Share - Earnings Per Share Calculation (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator for basic and diluted earnings per share – Net income available to common shareholders	$ 41,665	$ 29,784	$ 92,093	$ 66,262
Denominator for basic earnings per share – weighted average shares outstanding (in shares)	39,309	39,825	39,480	39,560
Effect of dilutive securities– Employee and director stock options and restricted share awards (in shares)	1,079	1,321	1,155	1,270
Denominator for diluted earnings per share – adjusted weighted average shares outstanding and assumed conversions (in shares)	40,388	41,146	40,635	40,830
Basic earnings per common share (in dollars per share)	$ 1.06	$ 0.75	$ 2.33	$ 1.68
Diluted earnings per common share and common share equivalents (in dollars per share)	$ 1.03	$ 0.72	$ 2.27	$ 1.62