Note 10 - Stock-based Compensation (Details Textual) - USD ($) $ in Millions | Sep. 03, 2019 | Jun. 03, 2019 | Jul. 13, 2018 | Jun. 30, 2019 | Sep. 30, 2019 | Sep. 30, 2018 | Dec. 31, 2018 |
Share-based Compensation Arrangement by Share-based Payment Award, Maximum Subscription Rate | | | | | 17.50% | | |
Share-based Compensation Arrangement by Share-based Payment Award, Maximum Number of Shares | | | | | 9,700,000 | | |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period | | | | | 4 years | | |
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period | | | | | 10 years | | |
Share-based Payment Arrangement, Nonvested Award, Option, Cost Not yet Recognized, Amount | | | | | $ 1.7 | | |
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition | | | | | 1 year 237 days | | |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number, Ending Balance | | | | | 6,104,000 | | 4,489,000 |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period | | | | | 80,000 | 150,000 | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period | | | | | 40,000 | | |
Employee Stock Option Vesting Schedule I [Member] | | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number, Ending Balance | | | | | 160,000 | 166,000 | |
Employee Stock Option Vesting Schedule II [Member] | | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage | | | | 100.00% | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number, Ending Balance | | | | | 335,000 | 850,000 | |
Restricted Stock Units (RSUs) [Member] | | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period | | | 90 days | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period | | 80,000 | 150,000 | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period | 40,000 | | | | | | |
Share-based Payment Arrangement, Tranche One [Member] | | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage | | | | | 50.00% | 50.00% | |
Share-based Payment Arrangement, Tranche One [Member] | Employee Stock Option Vesting Schedule I [Member] | | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage | | | | | 50.00% | 50.00% | |
Share-based Payment Arrangement, Tranche One [Member] | Restricted Stock Units (RSUs) [Member] | | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period | | 90 days | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage | 50.00% | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period | | 40,000 | | | | | |
Share-based Payment Arrangement, Tranche Two [Member] | | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage | | | | | 16.67% | 16.67% | |
Share-based Payment Arrangement, Tranche Two [Member] | Employee Stock Option Vesting Schedule II [Member] | | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage | | | | | 25.00% | 25.00% | |
Share-based Payment Arrangement, Tranche Two [Member] | Restricted Stock Units (RSUs) [Member] | | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period | | 1 year | | | | | |
Share-based Payment Arrangement, Tranche Three [Member] | | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage | | | | | 16.67% | 16.67% | |
Share-based Payment Arrangement, Tranche Three [Member] | Employee Stock Option Vesting Schedule II [Member] | | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage | | | | | | 25.00% | |
Share-based Payment Arrangement, Tranche Three [Member] | Employee Stock Option Vesting Schedule III [Member] | | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage | | | | | 25.00% | | |
Share-based Compensation Award, Tranche Four [Member] | | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Rights, Percentage | | | | | 16.67% | 16.67% | |