Note 14 - Stock-based Compensation - SARs Activity (Details) - Stock Appreciation Rights (SARs) [Member] - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Unvested (in shares)	731	704
Outstanding, aggregate intrinsic value	$ 8,639	$ 8,422	$ 12,923
Outstanding, weighted average fair value (in dollars per share)	$ 3.98	$ 3.82	$ 3.47
Outstanding, weighted average remaining contractual life (Year)	5 years 146 days	5 years 73 days	5 years 36 days
Awarded (in shares)	40	100
Shares awarded, grant date weighted average cost (in dollars per share)	$ 6.61	$ 6.07
Exercised (in shares)	(26)	(73)
Exercised, aggregate intrinsic value	$ 529	$ 1,654
Exercised, weighted average fair value (in dollars per share)	$ 3.55	$ 3.43
Forfeited (in shares)
Unvested (in shares)	745	731	704
Vested and exercisable (in shares)	535
Vested and exercisable, aggregate intrinsic value	$ 8,169
Vested and exercisable, weighted average fair value (in dollars per share)	$ 3.35
Vested and exercisable, weighted average remaining contractual life (Year)	4 years 36 days
Unvested (in shares)	210
Unvested, aggregate intrinsic value	$ 470
Unvested, weighted average fair value (in dollars per share)	$ 5.60
Unvested, weighted average remaining contractual life (Year)	8 years 219 days
Vested (in shares)	69
Vested, aggregate intrinsic value	$ 574
Vested, weighted average fair value (in dollars per share)	$ 4.44
Minimum [Member]
Outstanding, exercise price (in dollars per share)	14.02	$ 14.02
Granted, exercise price (in dollars per share)	36.65	35.90
Exercised, exercise price (in dollars per share)	14.02	14.02
Outstanding, exercise price (in dollars per share)	14.02	14.02	$ 14.02
Vested and exercisable, exercise price (in dollars per share)	14.02
Unvested, exercise price (in dollars per share)	19.44
Vested, exercise price (in dollars per share)	19.37
Maximum [Member]
Outstanding, exercise price (in dollars per share)	40	40
Granted, exercise price (in dollars per share)	36.65	39.32
Exercised, exercise price (in dollars per share)	19.37	19.37
Outstanding, exercise price (in dollars per share)	40	40	40
Vested and exercisable, exercise price (in dollars per share)	40
Unvested, exercise price (in dollars per share)	40
Vested, exercise price (in dollars per share)	40
Weighted Average [Member]
Outstanding, exercise price (in dollars per share)	22.42	19.51
Granted, exercise price (in dollars per share)	36.65	37.75
Exercised, exercise price (in dollars per share)	14.71	15.32
Outstanding, exercise price (in dollars per share)	23.45	$ 22.42	$ 19.51
Vested and exercisable, exercise price (in dollars per share)	19.01
Unvested, exercise price (in dollars per share)	34.78
Vested, exercise price (in dollars per share)	$ 26.77