Note 8 - Stock-based Compensation (Details Textual) - $ / shares | 3 Months Ended |
Mar. 31, 2021 | Mar. 31, 2020 |
Service Based Restricted Stock [Member] | Employee [Member] | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) | | 207,787 |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value (in dollars per share) | | $ 21.69 |
Service Based Restricted Stock [Member] | Employee [Member] | Share-based Payment Arrangement, Tranche One [Member] | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) | | 69,262 |
Service Based Restricted Stock [Member] | Employee [Member] | Share-based Payment Arrangement, Tranche Two [Member] | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) | | 69,262 |
Service Based Restricted Stock [Member] | Employee [Member] | Share-based Payment Arrangement, Tranche Three [Member] | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) | | 69,263 |
Service Based Restricted Stock [Member] | Employee [Member] | Common Class A [Member] | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) | 96,355 | 83,407 |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value (in dollars per share) | $ 17.20 | $ 19.87 |
Service Based Restricted Stock [Member] | Employee [Member] | Common Class A [Member] | Share-based Payment Arrangement, Tranche One [Member] | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) | | 27,802 |
Service Based Restricted Stock [Member] | Employee [Member] | Common Class A [Member] | Share-based Payment Arrangement, Tranche Two [Member] | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) | | 27,802 |
Service Based Restricted Stock [Member] | Employee [Member] | Common Class A [Member] | Share-based Payment Arrangement, Tranche Three [Member] | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) | | 27,803 |
Restricted Stock Units (RSUs) [Member] | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) | 95,115 | 90,184 |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value (in dollars per share) | $ 19.05 | $ 18.92 |
Restricted Stock Units (RSUs) [Member] | Employee [Member] | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) | 48,545 | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value (in dollars per share) | $ 18.21 | |
Restricted Stock Units (RSUs) [Member] | Nonemployee Directors [Member] | Share-based Payment Arrangement, Tranche One [Member] | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) | 32,118 | |
Restricted Stock Units (RSUs) [Member] | Nonemployee Directors [Member] | Share-based Payment Arrangement, Tranche Two [Member] | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) | 32,119 | |
Restricted Stock Units (RSUs) [Member] | Certain Employees [Member] | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) | | 90,184 |
Restricted Stock Units (RSUs) [Member] | Certain Employees [Member] | Share-based Payment Arrangement, Tranche One [Member] | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) | 82,499 | 60,052 |
Restricted Stock Units (RSUs) [Member] | Certain Employees [Member] | Share-based Payment Arrangement, Tranche Two [Member] | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) | | 15,066 |
Restricted Stock Units (RSUs) [Member] | Certain Employees One [Member] | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) | 95,115 | |
Performance Based Restricted Stock [Member] | Employee [Member] | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) | | 40,756 |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value (in dollars per share) | | $ 21.69 |
Performance Based Restricted Stock [Member] | Employee [Member] | Common Class A [Member] | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) | 96,355 | 83,407 |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value (in dollars per share) | $ 17.20 | $ 19.87 |
Performance Based Restricted Stock [Member] | Certain Employees [Member] | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) | 247,497 | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value (in dollars per share) | $ 18.21 | |
Employee [Member] | Performance Based Restricted Stock [Member] | Common Class A [Member] | Share-based Payment Arrangement, Tranche One [Member] | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) | 40,715 | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value (in dollars per share) | $ 19.87 | |