Note 8 - Stock-based Compensation - RSU Summary (Details) - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Beginning balance (in shares)	4,053,000	4,633,000	4,144,000
Weighted-Average Grant Date Fair Value Per Share, beginning balance (in dollars per share)	$ 59.16	$ 50.94	$ 39.91
Granted (in shares)	703,000	564,000	1,363,000
Weighted-Average Grant Date Fair Value Per Share, granted (in dollars per share)	$ 175.12	$ 103.68	$ 80.62
Vested (in shares)	(1,623,000)	(1,083,000)	(853,000)
Weighted-Average Grant Date Fair Value Per Share, vested (in dollars per share)	$ 54.70	$ 47.34	$ 44.13
Forfeited (in shares)	(28,000)	(61,000)	(21,000)
Weighted-Average Grant Date Fair Value Per Share, forfeited (in dollars per share)	$ 96.35	$ 57.01	$ 76.92
Ending balance (in shares)	3,105,000	4,053,000	4,633,000
Weighted-Average Grant Date Fair Value Per Share, ending balance (in dollars per share)	$ 87.42	$ 59.16	$ 50.94
Time-Based RSUs [Member]
Beginning balance (in shares)	180,000	240,000	258,000
Weighted-Average Grant Date Fair Value Per Share, beginning balance (in dollars per share)	$ 115.45	$ 95.38	$ 66.30
Granted (in shares)	76,000	52,000	133,000
Weighted-Average Grant Date Fair Value Per Share, granted (in dollars per share)	$ 189.28	$ 141.32	$ 114.36
Vested (in shares)	(86,000)	(103,000)	(136,000)
Weighted-Average Grant Date Fair Value Per Share, vested (in dollars per share)	$ 110.67	$ 81.53	$ 60.23
Forfeited (in shares)	(9,000)	(9,000)	(15,000)
Weighted-Average Grant Date Fair Value Per Share, forfeited (in dollars per share)	$ 138.34	$ 117.31	$ 82.20
Ending balance (in shares)	161,000	180,000	240,000
Weighted-Average Grant Date Fair Value Per Share, ending balance (in dollars per share)	$ 151.62	$ 115.45	$ 95.38
PSUs and MPSUs [Member]
Beginning balance (in shares)	1,987,000	2,174,000	2,266,000
Weighted-Average Grant Date Fair Value Per Share, beginning balance (in dollars per share)	$ 74.50	$ 61.61	$ 48.59
Granted (in shares)	627,000	512,000	630,000	[1]
Weighted-Average Grant Date Fair Value Per Share, granted (in dollars per share)	$ 173.40	$ 99.88	$ 85.06
Vested (in shares)	(1,213,000)	(656,000)	(717,000)
Weighted-Average Grant Date Fair Value Per Share, vested (in dollars per share)	$ 59.03	$ 53.72	$ 41.08
Forfeited (in shares)	(11,000)	(43,000)	(5,000)
Weighted-Average Grant Date Fair Value Per Share, forfeited (in dollars per share)	$ 84.48	$ 42.72	$ 63.16
Ending balance (in shares)	1,390,000	1,987,000	2,174,000
Weighted-Average Grant Date Fair Value Per Share, ending balance (in dollars per share)	$ 132.60	$ 74.50	$ 61.61
MSUs [Member]
Beginning balance (in shares)	1,886,000	2,219,000	1,620,000
Weighted-Average Grant Date Fair Value Per Share, beginning balance (in dollars per share)	$ 37.63	$ 35.69	$ 23.57
Granted (in shares)	0	0	600,000
Weighted-Average Grant Date Fair Value Per Share, granted (in dollars per share)	$ 0	$ 0	$ 68.48
Vested (in shares)	(324,000)	(324,000)	0
Weighted-Average Grant Date Fair Value Per Share, vested (in dollars per share)	$ 23.57	$ 23.57	$ 0
Forfeited (in shares)	(8,000)	(9,000)	(1,000)
Weighted-Average Grant Date Fair Value Per Share, forfeited (in dollars per share)	$ 68.48	$ 68.48	$ 68.48
Ending balance (in shares)	1,554,000	1,886,000	2,219,000
Weighted-Average Grant Date Fair Value Per Share, ending balance (in dollars per share)	$ 40.40	$ 37.63	$ 35.69

[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.