Note 8 - Stock-based Compensation - Restricted Stock Options (Details) - Restricted Stock [Member]	3 Months Ended
	Mar. 31, 2020 $ / shares shares
Restricted stock (in shares) | shares	314,973
Restricted stock, weighted-average grant date fair value (in dollars per share) | $ / shares	$ 89.55
Granted (in shares) | shares	28,105
Granted, weighted-average grant date fair value (in dollars per share) | $ / shares	$ 96.72
Restrictions lapsed (in shares) | shares	(25,598)
Restrictions lapsed, weighted-average grant date fair value (in dollars per share) | $ / shares	$ 79.27
Forfeited (in shares) | shares	(7,616)
Forfeited, weighted-average grant date fair value (in dollars per share) | $ / shares	$ 90.47
Restricted stock (in shares) | shares	309,864
Restricted stock, weighted-average grant date fair value (in dollars per share) | $ / shares	$ 91.03