Note 7 - Supplemental Equity and Accumulated Other Comprehensive Income (Loss) (Details Textual) - USD ($) $ in Thousands	6 Months Ended		12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2017	Jun. 30, 2017
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	$ 0	$ 0
Stockholders' Equity Attributable to Parent, Ending Balance	1,096,425	997,819		$ 1,109,469	$ 1,079,061	$ 960,082	$ 949,627
Accumulated Other Comprehensive Income (Loss), Derivative Qualifying as Hedge, Excluded Component, Including Portion Attributable to Noncontrolling Interest [Member]
Stockholders' Equity Attributable to Parent, Ending Balance	$ (3,915)
Scenario, Forecast [Member]
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent			$ 250