Note 8 - Stock-based Compensation - Non-vested Restricted Stock Options (Details) - Restricted Stock [Member]	6 Months Ended
	Jun. 30, 2017 $ / shares shares
Restricted stock, shares (in shares) | shares	215,105
Restricted stock, weighted-average grant date fair value per share (in dollars per share) | $ / shares	$ 62.78
Restricted stock, weighted-average grant date fair value per share (in dollars per share) | $ / shares	$ 61.95
Granted, shares (in shares) | shares	197,649
Granted, weighted-average grant date fair value per share (in dollars per share) | $ / shares	$ 61.60
Restrictions lapsed, shares (in shares) | shares	(55,139)
Restrictions lapsed, weighted-average grant date fair value per share (in dollars per share) | $ / shares	$ 63.61
Forfeited, shares (in shares) | shares	(9,570)
Forfeited, weighted-average grant date fair value per share (in dollars per share) | $ / shares	$ 63.76
Restricted stock, shares (in shares) | shares	348,045