Note 5 - Share-based Compensation - Stock Option Activity Under Equity Incentive Plans and Director Plan (Details) - USD ($)	3 Months Ended				6 Months Ended		12 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Exercised, number of options (in shares)	(148,284)	(260,481)	(18,000)	(86,247)
Common Stock Options [Member]
Outstanding, number of options (in shares)		1,245,922			1,245,922
Outstanding, weighted average exercise price (in dollars per share)		$ 18.08			$ 18.08
Outstanding, weighted average remaining contractual term (Year)					5 years 9 months 7 days		4 years 5 months 12 days
Granted, number of options (in shares)					70,471	100,615
Granted, weighted average exercise price (in dollars per share)					$ 62.23
Exercised, number of options (in shares)					(408,765)
Exercised, weighted average exercise price (in dollars per share)					$ 12.68
Forfeited, number of options (in shares)					(15,490)
Forfeited, weighted average exercise price (in dollars per share)					$ 48.42
Outstanding, number of options (in shares)	892,138				892,138		1,245,922
Outstanding, weighted average exercise price (in dollars per share)	$ 23.51				$ 23.51		$ 18.08
Exercisable, number of options (in shares)	492,484				492,484
Weighted average exercise price (in dollars per share)	$ 17.55				$ 17.55
Exercisable, weighted average remaining contractual term (Year)					4 years 6 months 29 days
Common Stock Options [Member] | Common Stock [Member]
Outstanding, aggregate intrinsic value	$ 31,281,000				$ 31,281,000		$ 59,631,000
Exercised, aggregate intrinsic value					17,810,000
Exercisable, aggregate intrinsic value	$ 20,022.000				$ 20,022.000