Note 8 - Stock-based Compensation - RSU Summary (Details) - $ / shares		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Beginning balance (in shares)		4,633,000	4,144,000	4,270,000
Weighted-Average Grant Date Fair Value Per Share, beginning balance (in dollars per share)		$ 50.94	$ 39.91	$ 36.47
Granted (in shares)		564,000	1,363,000	666,000
Weighted-Average Grant Date Fair Value Per Share, granted (in dollars per share)		$ 103.68	$ 80.62	$ 66.56
Vested (in shares)		(1,083,000)	(853,000)	(772,000)
Weighted-Average Grant Date Fair Value Per Share, vested (in dollars per share)		$ 47.34	$ 44.13	$ 43.39
Forfeited (in shares)		(61,000)	(21,000)	(20,000)
Weighted-Average Grant Date Fair Value Per Share, forfeited (in dollars per share)		$ 57.01	$ 76.92	$ 58.46
Ending balance (in shares)		4,053,000	4,633,000	4,144,000
Weighted-Average Grant Date Fair Value Per Share, ending balance (in dollars per share)		$ 59.16	$ 50.94	$ 39.91
Time-Based RSUs [Member]
Beginning balance (in shares)		240,000	258,000	366,000
Weighted-Average Grant Date Fair Value Per Share, beginning balance (in dollars per share)		$ 95.38	$ 66.30	$ 51.35
Granted (in shares)		52,000	133,000	81,000
Weighted-Average Grant Date Fair Value Per Share, granted (in dollars per share)		$ 141.32	$ 114.36	$ 94.25
Vested (in shares)		(103,000)	(136,000)	(175,000)
Weighted-Average Grant Date Fair Value Per Share, vested (in dollars per share)		$ 81.53	$ 60.23	$ 48.35
Forfeited (in shares)		(9,000)	(15,000)	(14,000)
Weighted-Average Grant Date Fair Value Per Share, forfeited (in dollars per share)		$ 117.31	$ 82.20	$ 61.80
Ending balance (in shares)		180,000	240,000	258,000
Weighted-Average Grant Date Fair Value Per Share, ending balance (in dollars per share)		$ 115.45	$ 95.38	$ 66.30
PSUs and MPSUs [Member]
Beginning balance (in shares)		2,174,000	2,266,000	2,284,000
Weighted-Average Grant Date Fair Value Per Share, beginning balance (in dollars per share)		$ 61.61	$ 48.59	$ 43.24
Granted (in shares)	[1]	512,000	630,000	585,000
Weighted-Average Grant Date Fair Value Per Share, granted (in dollars per share)		$ 99.88	$ 85.06	$ 62.72
Vested (in shares)		(656,000)	(717,000)	(597,000)
Weighted-Average Grant Date Fair Value Per Share, vested (in dollars per share)		$ 53.72	$ 41.08	$ 41.94
Forfeited (in shares)		(43,000)	(5,000)	(6,000)
Weighted-Average Grant Date Fair Value Per Share, forfeited (in dollars per share)		$ 42.72	$ 63.16	$ 49.82
Ending balance (in shares)		1,987,000	2,174,000	2,266,000
Weighted-Average Grant Date Fair Value Per Share, ending balance (in dollars per share)		$ 74.50	$ 61.61	$ 48.59
MSUs [Member]
Beginning balance (in shares)		2,219,000	1,620,000	1,620,000
Weighted-Average Grant Date Fair Value Per Share, beginning balance (in dollars per share)		$ 35.69	$ 23.57	$ 23.57
Granted (in shares)		0	600,000	0
Weighted-Average Grant Date Fair Value Per Share, granted (in dollars per share)		$ 0	$ 68.48	$ 0
Vested (in shares)		(324,000)	0	0
Weighted-Average Grant Date Fair Value Per Share, vested (in dollars per share)		$ 23.57	$ 0	$ 0
Forfeited (in shares)		(9,000)	(1,000)	0
Weighted-Average Grant Date Fair Value Per Share, forfeited (in dollars per share)		$ 68.48	$ 68.48	$ 0
Ending balance (in shares)		1,886,000	2,219,000	1,620,000
Weighted-Average Grant Date Fair Value Per Share, ending balance (in dollars per share)		$ 37.63	$ 35.69	$ 23.57

[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.