Note 3 - Stock-based Compensation - RSU Summary (Details)	3 Months Ended
	Mar. 31, 2021 $ / shares shares
Beginning balance (in shares) | shares	3,105,000
Weighted-Average Grant Date Fair Value Per Share, beginning balance (in dollars per share) | $ / shares	$ 87.42
Granted (in shares) | shares	387,000
Weighted-Average Grant Date Fair Value Per Share, granted (in dollars per share) | $ / shares	$ 353.33
Vested (in shares) | shares	(460,000)
Weighted-Average Grant Date Fair Value Per Share, vested (in dollars per share) | $ / shares	$ 82.43
Forfeited (in shares) | shares	(12,000)
Weighted-Average Grant Date Fair Value Per Share, forfeited (in dollars per share) | $ / shares	$ 94.88
Ending balance (in shares) | shares	3,020,000
Weighted-Average Grant Date Fair Value Per Share, ending balance (in dollars per share) | $ / shares	$ 122.26
Time-Based RSUs [Member]
Beginning balance (in shares) | shares	161,000
Weighted-Average Grant Date Fair Value Per Share, beginning balance (in dollars per share) | $ / shares	$ 151.62
Granted (in shares) | shares	25,000
Weighted-Average Grant Date Fair Value Per Share, granted (in dollars per share) | $ / shares	$ 374.57
Vested (in shares) | shares	(21,000)
Weighted-Average Grant Date Fair Value Per Share, vested (in dollars per share) | $ / shares	$ 139.43
Forfeited (in shares) | shares	(1,000)
Weighted-Average Grant Date Fair Value Per Share, forfeited (in dollars per share) | $ / shares	$ 163.87
Ending balance (in shares) | shares	164,000
Weighted-Average Grant Date Fair Value Per Share, ending balance (in dollars per share) | $ / shares	$ 187.34
PSUs and MPSUs [Member]
Beginning balance (in shares) | shares	1,390,000
Weighted-Average Grant Date Fair Value Per Share, beginning balance (in dollars per share) | $ / shares	$ 132.60
Granted (in shares) | shares	362,000	[1]
Weighted-Average Grant Date Fair Value Per Share, granted (in dollars per share) | $ / shares	$ 351.85
Vested (in shares) | shares	(358,000)
Weighted-Average Grant Date Fair Value Per Share, vested (in dollars per share) | $ / shares	$ 92.39
Forfeited (in shares) | shares	(10,000)
Weighted-Average Grant Date Fair Value Per Share, forfeited (in dollars per share) | $ / shares	$ 86.01
Ending balance (in shares) | shares	1,384,000
Weighted-Average Grant Date Fair Value Per Share, ending balance (in dollars per share) | $ / shares	$ 200.75
MSUs [Member]
Beginning balance (in shares) | shares	1,554,000
Weighted-Average Grant Date Fair Value Per Share, beginning balance (in dollars per share) | $ / shares	$ 40.40
Granted (in shares) | shares	0
Weighted-Average Grant Date Fair Value Per Share, granted (in dollars per share) | $ / shares	$ 0
Vested (in shares) | shares	(81,000)
Weighted-Average Grant Date Fair Value Per Share, vested (in dollars per share) | $ / shares	$ 23.57
Forfeited (in shares) | shares	(1,000)
Weighted-Average Grant Date Fair Value Per Share, forfeited (in dollars per share) | $ / shares	$ 68.48
Ending balance (in shares) | shares	1,472,000
Weighted-Average Grant Date Fair Value Per Share, ending balance (in dollars per share) | $ / shares	$ 41.31

[1] Amount reflects the number of awards that may ultimately be earned based on management’s probability assessment of the achievement of performance conditions at each reporting period.