Note 2 - New Accounting Pronouncements (Details Textual) - USD ($) $ in Thousands	Jun. 30, 2020	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Ending Balance	$ 1,556,811	$ 1,533,651		$ 1,515,410	$ 1,491,641	$ 1,462,995	$ 1,445,096
Cumulative Effect, Period of Adoption, Adjustment [Member]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Ending Balance				(755)			(58)
Retained Earnings [Member]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Ending Balance	$ 524,864	$ 507,537		487,873	$ 470,880	$ 442,531	422,222
Retained Earnings [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Ending Balance				$ (755)			$ (58)
Accounting Standards Update 2016-13 [Member] | Retained Earnings [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Ending Balance			$ (800)