Note 6 - Share-based Payments - Stock Incentive Awards Granted to Officers Under the 2014 Stock Incentive Plan (Details)	9 Months Ended
	Sep. 30, 2019 $ / shares shares
Performance Shares [Member] | The 2014 Stock Incentive Plan [Member] | February 13, 2019 [Member]
Shares/units granted (in shares) | shares	47,800
Shares/units granted, weighted average grant-date fair value per award (in dollars per share) | $ / shares	$ 42.875
Shares/units granted, vesting date	December 31, 2021
Performance Shares [Member] | The 2014 Stock Incentive Plan Under Legacy Agreement [Member]
Shares/units granted, vesting date	December 31, 2021
Performance Shares [Member] | The 2014 Stock Incentive Plan Under Legacy Agreement [Member] | February 13, 2019 [Member]
Shares/units granted (in shares) | shares	7,800
Shares/units granted, weighted average grant-date fair value per award (in dollars per share) | $ / shares	$ 45.885
Restricted Stock Units (RSUs) [Member] | The 2014 Stock Incentive Plan [Member] | February 13, 2019 [Member] | Executive Officer [Member]
Shares/units granted (in shares) | shares	15,600
Shares/units granted, weighted average grant-date fair value per award (in dollars per share) | $ / shares	$ 49.6225
Shares/units granted, vesting date	25% per year through February 6, 2023
Restricted Stock Units (RSUs) [Member] | The 2014 Stock Incentive Plan [Member] | April 8, 2019 [Member] | Key Employee [Member]
Shares/units granted (in shares) | shares	13,270
Shares/units granted, weighted average grant-date fair value per award (in dollars per share) | $ / shares	$ 44.45
Shares/units granted, vesting date	100% on April 8, 2023
Restricted Stock [Member] | The 2014 Stock Incentive Plan [Member] | April 8, 2019 [Member] | Nonemployee Directors [Member]
Shares/units granted (in shares) | shares	15,700
Shares/units granted, weighted average grant-date fair value per award (in dollars per share) | $ / shares	$ 49.73
Shares/units granted, vesting date	33% per year through April 8, 2022