Note 7 - Equity Incentive Plans - Options Activity (Details)	6 Months Ended
	Jun. 30, 2020 $ / shares shares
Service Period Stock Options [Member]
Outstanding at December 31, 2019 (in shares) | shares	1,359,990
Outstanding at December 31, 2019, weighted average exercise price (in dollars per share) | $ / shares	$ 11.50
Granted (in shares) | shares	54,782
Granted, weighted average exercise price (in dollars per share) | $ / shares	$ 62.84
Exercised (in shares) | shares	(240,265)
Exercised, weighted average exercise price (in dollars per share) | $ / shares	$ 8.45
Forfeited (in shares) | shares	(357)
Forfeited, weighted average exercise price (in dollars per share) | $ / shares	$ 11.00
Outstanding at June 30, 2020 (in shares) | shares	1,174,150
Outstanding at June 30, 2020, weighted average exercise price (in dollars per share) | $ / shares	$ 14.52
Performance Shares [Member]
Outstanding at December 31, 2019 (in shares) | shares	1,372,819
Outstanding at December 31, 2019, weighted average exercise price (in dollars per share) | $ / shares	$ 18.31
Granted (in shares) | shares	19,317
Granted, weighted average exercise price (in dollars per share) | $ / shares	$ 63.87
Exercised (in shares) | shares	(3,687)
Exercised, weighted average exercise price (in dollars per share) | $ / shares	$ 16.45
Outstanding at June 30, 2020 (in shares) | shares	1,388,449
Outstanding at June 30, 2020, weighted average exercise price (in dollars per share) | $ / shares	$ 18.95