Note 7 - Stock-based Compensation (Details Textual) - USD ($) $ / shares in Units, $ in Millions | Feb. 01, 2019 | Jan. 02, 2019 | Sep. 30, 2018 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 |
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Total | | | | $ 4.2 | | |
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition | | | | 1 year 182 days | | |
Restricted Stock [Member] | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period | | | | 391,836 | 307,943 | 237,500 |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value | | | | $ 14.63 | $ 10.40 | $ 12.88 |
Restricted Stock [Member] | Employee [Member] | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period | | | | 318,196 | 307,943 | 218,452 |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value | | | | $ 15.25 | | |
Restricted Stock [Member] | Employee [Member] | Subsequent Event [Member] | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period | 336,609 | 340,993 | | | | |
Stock Granted, Value, Share-based Compensation, Net of Forfeitures, Total | $ 5.6 | $ 5.1 | | | | |
Restricted Stock [Member] | Employee [Member] | Share-based Compensation Award, Tranche One [Member] | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period | | | | 131,106 | | |
Restricted Stock [Member] | Employee [Member] | Share-based Compensation Award, Tranche Two [Member] | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period | | | | 69,651 | | |
Restricted Stock [Member] | Employee [Member] | Share-based Compensation Award, Tranche Three [Member] | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period | | | | 69,652 | | |
Restricted Stock [Member] | Employee [Member] | Share-based Compensation Award, Tranche One, Vesting January 31, 2018 [Member] | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period | | | | | 102,648 | |
Restricted Stock [Member] | Employee [Member] | Share-based Compensation Award, Tranche Two, Vesting January 31, 2019 [Member] | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period | | | | | 102,648 | 72,820 |
Restricted Stock [Member] | Employee [Member] | Share-based Compensation Award, Tranche Three, Vesting January 31, 2020 [Member] | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period | | | | | 102,647 | |
Restricted Stock [Member] | Employee [Member] | Share-based Compensation Award, Tranche One, Vesting January 31, 2017 [Member] | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period | | | | | | 72,816 |
Restricted Stock [Member] | Employee [Member] | Share-based Compensation Award, Tranche Three, Vesting January 31, 2019 [Member] | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period | | | | | | 72,820 |
Restricted Stock [Member] | Director [Member] | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period | | | | 73,640 | 76,856 | 19,048 |
Restricted Stock [Member] | Common Class A [Member] | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period | | | | 220,080 | 275,076 | 218,612 |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value | | | | $ 12.65 | $ 10.84 | $ 11.25 |
Restricted Stock [Member] | Common Class A [Member] | Employee [Member] | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period | | | | | 198,220 | 166,677 |
Restricted Stock [Member] | Common Class A [Member] | Employee [Member] | Subsequent Event [Member] | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period | 199,810 | | | | | |
Stock Granted, Value, Share-based Compensation, Net of Forfeitures, Total | $ 3.1 | | | | | |
Restricted Stock [Member] | Common Class A [Member] | Employee [Member] | Share-based Compensation Award, Tranche One, Vesting January 31, 2018 [Member] | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period | | | | | 66,073 | |
Restricted Stock [Member] | Common Class A [Member] | Employee [Member] | Share-based Compensation Award, Tranche Two, Vesting January 31, 2019 [Member] | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period | | | | | 66,073 | |
Restricted Stock [Member] | Common Class A [Member] | Employee [Member] | Share-based Compensation Award, Tranche Three, Vesting January 31, 2020 [Member] | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period | | | | | 66,074 | |
Restricted Stock [Member] | Common Class A [Member] | Employee [Member] | Share-based Compensation Award, Tranche One, Vesting January 31, 2017 [Member] | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period | | | | | | 55,559 |
Restricted Stock [Member] | Common Class A [Member] | Employee [Member] | Share-based Compensation Award, Tranche Three, Vesting January 31, 2019 [Member] | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period | | | | | | 55,559 |
Restricted Stock [Member] | Common Class A [Member] | Employee [Member] | Share-based Compensation Award, Tranche Two, Vesting January 31, 2018 [Member] | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period | | | | | | 55,559 |
Restricted Stock [Member] | Common Class A [Member] | Director [Member] | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period | | | | | | 51,935 |
Service Based Restricted Stock [Member] | Common Class A [Member] | Employee [Member] | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period | | | | 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 | | | | $ 12.65 | | |
Service Based Restricted Stock [Member] | Common Class A [Member] | Employee [Member] | Share-based Compensation Award, Tranche One [Member] | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period | | | | 36,680 | | |
Service Based Restricted Stock [Member] | Common Class A [Member] | Employee [Member] | Share-based Compensation Award, Tranche Two [Member] | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period | | | | 36,680 | | |
Service Based Restricted Stock [Member] | Common Class A [Member] | Employee [Member] | Share-based Compensation Award, Tranche Three [Member] | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period | | | 36,680 | | | |
Performance Based Restricted Stock [Member] | Common Class A [Member] | Employee [Member] | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period | | | | 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 | | | | $ 12.65 | | |
Restricted Stock Units (RSUs) [Member] | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period | | | | | 215,500 | 0 |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value | | | | | $ 15.70 | $ 0 |
Restricted Stock Units (RSUs) [Member] | Share-based Compensation Award, Tranche One, Vesting January 31, 2018 [Member] | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period | | | | | 209,500 | |
Directors Restricted Stock Plan [Member] | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award Maximum Number of Restricted Shares Authorized Yearly Per Director | | | | 10,000 | | |
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant | | | | 770,000 | | |
Directors Restricted Stock Plan [Member] | Restricted Stock [Member] | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period | | | | 0 | 0 | 0 |
The 2017 Equity and Incentive Compensation Plan [Member] | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant | | | | 4,400,000 | | |
The 2017 Equity and Incentive Compensation Plan [Member] | Common Class A [Member] | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant | | | | 1,700,000 | | |
2007 Incentive Plan [Member] | | | | | | |
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant | | | | 0 | 0 | |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number, Ending Balance | | | | 274,746 | 274,746 | |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price, Ending Balance | | | | $ 1.99 | $ 1.99 | |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value | | | | $ 3.5 | | |