Note 7 - Shareholders' Equity - Performance Shares Outstanding (Details) - Performance Shares [Member]	3 Months Ended
	Mar. 31, 2019 $ / shares shares
Outstanding (in shares) | shares	759,500
Outstanding, weighted average grant date fair value (in dollars per share) | $ / shares	$ 20.17
Granted (in shares) | shares	344,500
Granted, weighted average grant date fair value (in dollars per share) | $ / shares	$ 17.54
Vested (in shares) | shares	470,500
Vested, weighted average grant date fair value (in dollars per share) | $ / shares	$ 18.84
Forfeited or canceled (in shares) | shares	34,500
Forfeited or canceled, weighted average grant date fair value (in dollars per share) | $ / shares	$ 20.59
Outstanding (in shares) | shares	599,000
Outstanding, weighted average grant date fair value (in dollars per share) | $ / shares	$ 19.67