Note 4 - Stock-based Compensation - Changes in Retention Awards (Details) - Retention Awards [Member] shares in Thousands	6 Months Ended
	Jun. 30, 2022 $ / shares shares
Nonvested, beginning balance (in shares) | shares	1,287
Nonvested, weighted-average grant-date fair value, beginning balance (in dollars per share) | $ / shares	$ 165.10
Granted (in shares) | shares	234
Granted, weighted-average grant-date fair value (in dollars per share) | $ / shares	$ 244.27
Vested (in shares) | shares	(408)
Vested, weighted-average grant-date fair value (in dollars per share) | $ / shares	$ 125.98
Forfeited (in shares) | shares	(36)
Forfeited, weighted-average grant-date fair value (in dollars per share) | $ / shares	$ 190.72
Nonvested, ending balance (in shares) | shares	1,077
Nonvested, weighted-average grant-date fair value, ending balance (in dollars per share) | $ / shares	$ 196.26