Note 9 - Stock Based Compensation - Summary of Stock Option Activity (Details) - Employee Stock Option [Member] - $ / shares	3 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017
Outstanding, beginning balance (in shares)	1,354,293	1,497,126	1,519,338
Outstanding, beginning balance, weighted average exercise price (in dollars per share)	$ 28.51	$ 26.52	$ 25.22
Granted (in shares)		19,598	39,037
Granted, weighted average exercise price (in dollars per share)		$ 58.98	$ 54.15
Exercised (in shares)	(55,950)	(162,431)	(61,249)
Exercised, weighted average exercise price (in dollars per share)	$ 8.48	$ 13.84	$ 11.83
Exercised, weighted average exercise price (in dollars per share)	$ 8.48	$ 13.84	$ 11.83
Outstanding, ending balance (in shares)	1,298,343	1,354,293	1,497,126
Outstanding, ending balance, weighted average exercise price (in dollars per share)	$ 29.37	$ 28.51	$ 26.52
Exercised (in shares)	449,959
Exercised, weighted average exercise price (in dollars per share)	$ 21.09