Note 4 - Stockholders' Equity - Summary of Restricted Stock Unit Activity (Details) - Restricted Stock Units (RSUs) [Member]	6 Months Ended
	Jun. 30, 2017 $ / shares shares
Unvested balance (in shares) | shares	1,399,127
Unvested balance (in dollars per share) | $ / shares	$ 19.96
Unvested balance (in dollars per share) | $ / shares	$ 25.11
Granted (in shares) | shares	1,011,903
Granted (in dollars per share) | $ / shares	$ 28.93
Vested (in shares) | shares	(309,675)
Vested (in dollars per share) | $ / shares	$ 20.46
Forfeited (in shares) | shares	(153,200)
Forfeited (in dollars per share) | $ / shares	$ 21.40
Unvested balance (in shares) | shares	1,948,155