Note 8 - Stock-based Compensation (Details Textual) - USD ($) $ / shares in Units, $ in Millions | 9 Months Ended | 12 Months Ended |
Sep. 30, 2019 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 |
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount, Total | | $ 7 | | |
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition (Year) | | 1 year 6 months | | |
The 2017 Equity and Incentive Compensation Plan [Member] | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant (in shares) | | 3,000,000 | | |
Common Class A [Member] | The 2017 Equity and Incentive Compensation Plan [Member] | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant (in shares) | | 1,300,000 | | |
Restricted Stock [Member] | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) | | 359,481 | 751,463 | 391,836 |
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) | | $ 18.92 | $ 16.07 | $ 14.63 |
Restricted Stock [Member] | Common Class A [Member] | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) | | 166,814 | 199,810 | 220,080 |
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 | $ 15.36 | $ 12.65 |
Restricted Stock [Member] | Nonemployee Directors [Member] | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) | | 78,722 | 41,181 | 73,640 |
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) | | $ 11.56 | $ 22.10 | $ 11.95 |
Restricted Stock [Member] | Employee [Member] | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) | | 32,216 | | 318,196 |
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) | | $ 15.52 | | $ 15.25 |
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) | | | 11,223 | 131,106 |
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) | | | $ 17.83 | |
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) | | | 32,680 | 69,651 |
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) | | | $ 15.92 | |
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) | | | | 69,652 |
Restricted Stock [Member] | Certain Employees One [Member] | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) | | | 340,993 | |
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) | | | $ 14.85 | |
Restricted Stock [Member] | Certain Employees Two [Member] | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) | | | 277,048 | |
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) | | | $ 16.55 | |
Restricted Stock [Member] | Certain Employees Two [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) | | | 92,349 | |
Restricted Stock [Member] | Certain Employees Two [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) | | | 92,349 | |
Restricted Stock [Member] | Certain Employees Two [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) | | | 92,350 | |
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) | | 83,407 | 99,905 | 110,040 |
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 | $ 15.36 | $ 12.65 |
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 | 33,302 | 36,680 |
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 | 33,302 | 36,680 |
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 | 33,301 | |
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 | 48,338 | |
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 | $ 16.55 | |
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) | | 83,407 | 99,905 | 110,040 |
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 | $ 15.36 | $ 12.65 |
Restricted Stock Units (RSUs) [Member] | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period (in shares) | 398,000 | 93,184 | 398,000 | 0 |
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 | $ 18.77 | $ 18.21 | $ 0 |
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) | | 3,000 | | |
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) | | $ 14.30 | | |
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 | | |
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) | | $ 18.92 | | |
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) | | 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 [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) | | 15,066 | | |