Note 5 - Share-based Compensation - Stock Option Activity Under Equity Incentive Plans and Director Plan (Details) - USD ($)	3 Months Ended	12 Months Ended
Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Exercised, number of options (in shares)	(260,481)	(86,247)
Common Stock [Member]
Outstanding, number of options (in shares)	1,245,922
Outstanding, weighted average exercise price (in dollars per share)	$ 18.08
Outstanding, weighted average remaining contractual term (Year)	5 years 131 days		4 years 164 days
Outstanding, aggregate intrinsic value	$ 25,579,000		$ 59,631
Granted, number of options (in shares)	48,934
Granted, weighted average exercise price (in dollars per share)	$ 65.80
Exercised, number of options (in shares)	(260,481)
Exercised, weighted average exercise price (in dollars per share)	$ 12.08
Exercised, aggregate intrinsic value	$ 12,059,000
Forfeited, number of options (in shares)
Forfeited, weighted average exercise price (in dollars per share)
Outstanding, number of options (in shares)	1,034,375		1,245,922
Outstanding, weighted average exercise price (in dollars per share)	$ 21.85		$ 18.08
Exercisable, number of options (in shares)	613,484
Weighted average exercise price (in dollars per share)	$ 15.16
Exercisable, weighted average remaining contractual term (Year)	3 years 321 days
Exercisable, aggregate intrinsic value	$ 18,600,000

The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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For presentations that combine terminations, the number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan or that expired.

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Weighted average price of options that were either forfeited or expired.

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Gross number of share options (or share units) granted during the period.

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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Number of options outstanding, including both vested and non-vested options.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Number of share options (or share units) exercised during the current period.

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