Note 2 - Recently Issued Accounting Pronouncements (Details Textual) - USD ($) $ in Thousands	Jan. 01, 2022	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Nov. 30, 2019
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Ending Balance		$ 1,022,354	$ 988,242	$ 991,610	$ 988,999	$ 1,090,760	$ 1,178,416
Retained Earnings [Member]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Ending Balance		$ 430,074	$ 401,061	$ 424,835	$ 422,846	$ 520,025	$ 594,353
Retained Earnings [Member] | Revision of Prior Period, Adjustment [Member] | Accounting Standards Update 2020-06 [Member] | Forecast [Member] | Minimum [Member]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Ending Balance	$ 20,000
Retained Earnings [Member] | Revision of Prior Period, Adjustment [Member] | Accounting Standards Update 2020-06 [Member] | Forecast [Member] | Maximum [Member]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Ending Balance	$ 40,000
The 2.75% Convertible Notes [Member]
Debt Instrument, Interest Rate, Stated Percentage		2.75%		2.75%	2.75%			2.75%
Long-term Debt, Gross		$ 205,500		$ 200,300	$ 198,600			$ 192,600
Debt Instrument, Convertible, Carrying Amount of Equity Component		$ 24,500		$ 29,700	$ 31,400			27,900
Debt Instrument, Convertible, Carrying Amount of Equity Component, Tax Amount								$ 9,500