Note 8 - Stock-based Compensation (Details Textual) - USD ($)	3 Months Ended	6 Months Ended	114 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018
Allocated Share-based Compensation Expense, Total	$ 1,200,000	$ 1,000,000	$ 4,600,000	$ 4,100,000
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense	$ 300,000	$ 400,000	$ 1,000,000	$ 1,200,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross	600	0	600	4,400	0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Number	600		600		600
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Weighted Average Exercise Price	$ 8.44		$ 8.44		$ 8.44
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period	5,600	0	280,590	276,900
Restricted Stock Units (RSUs) [Member] \| Omnibus Plan 2014 [Member]
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Total	$ 10,800,000		$ 10,800,000		$ 10,800,000
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition			3 years 36 days
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value	28,000	$ 40,000	$ 6,700,000	$ 4,800,000
Phantom Share Units (PSUs) [Member] \| Phantom Stock Plan [Member]
Allocated Share-based Compensation Expense, Total	100,000	100,000	100,000	100,000
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value	$ 1,000	$ 1,000	$ 1,000	$ 6,000
Share-based Compensation Arrangement by Share-based Payment Award, Award Requisite Service Period			1 year

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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Unrecognized cost of unvested share-based compensation awards.

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Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the period.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

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Number of fully vested and expected to vest options outstanding that can be converted into shares under option plan. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted-average exercise price, at which grantee can acquire shares reserved for issuance, for fully vested and expected to vest options outstanding. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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