Note 3 - Stock-based Compensation - RSU Summary (Details) - $ / shares		3 Months Ended	12 Months Ended
		Mar. 31, 2019	Jan. 07, 2020
Beginning balance (in shares)		4,633,000	4,633,000
Weighted-Average Grant Date Fair Value Per Share, beginning balance (in dollars per share)		$ 50.94	$ 50.94
Granted (in shares)		337,000
Weighted-Average Grant Date Fair Value Per Share, granted (in dollars per share)		$ 108.93
Vested (in shares)		(514,000)
Weighted-Average Grant Date Fair Value Per Share, vested (in dollars per share)		$ 52.86
Forfeited (in shares)		(3,000)
Weighted-Average Grant Date Fair Value Per Share, forfeited (in dollars per share)		$ 83.26
Ending balance (in shares)		4,453,000
Weighted-Average Grant Date Fair Value Per Share, ending balance (in dollars per share)		$ 55.09
Time-Based RSUs [Member]
Beginning balance (in shares)		240,000	240,000
Weighted-Average Grant Date Fair Value Per Share, beginning balance (in dollars per share)		$ 95.38	$ 95.38
Granted (in shares)		26,000	26,000
Weighted-Average Grant Date Fair Value Per Share, granted (in dollars per share)		$ 130.67
Vested (in shares)		(33,000)
Weighted-Average Grant Date Fair Value Per Share, vested (in dollars per share)		$ 77.83
Forfeited (in shares)		(2,000)
Weighted-Average Grant Date Fair Value Per Share, forfeited (in dollars per share)		$ 89.22
Ending balance (in shares)		231,000
Weighted-Average Grant Date Fair Value Per Share, ending balance (in dollars per share)		$ 101.91
PSUs and MPSUs [Member]
Beginning balance (in shares)		2,174,000	2,174,000
Weighted-Average Grant Date Fair Value Per Share, beginning balance (in dollars per share)		$ 61.61	$ 61.61
Granted (in shares)	[1]	311,000
Weighted-Average Grant Date Fair Value Per Share, granted (in dollars per share)		$ 107.14
Vested (in shares)		(400,000)
Weighted-Average Grant Date Fair Value Per Share, vested (in dollars per share)		$ 56.71
Forfeited (in shares)
Weighted-Average Grant Date Fair Value Per Share, forfeited (in dollars per share)
Ending balance (in shares)		2,085,000
Weighted-Average Grant Date Fair Value Per Share, ending balance (in dollars per share)		$ 69.34
MSUs [Member]
Beginning balance (in shares)		2,219,000	2,219,000
Weighted-Average Grant Date Fair Value Per Share, beginning balance (in dollars per share)		$ 35.69	$ 35.69
Granted (in shares)	[1]
Weighted-Average Grant Date Fair Value Per Share, granted (in dollars per share)
Vested (in shares)		(81,000)
Weighted-Average Grant Date Fair Value Per Share, vested (in dollars per share)		$ 23.57
Forfeited (in shares)		(1,000)
Weighted-Average Grant Date Fair Value Per Share, forfeited (in dollars per share)		$ 68.48
Ending balance (in shares)		2,137,000
Weighted-Average Grant Date Fair Value Per Share, ending balance (in dollars per share)		$ 36.14

[1] Amount reflects the number of PSUs and MPSUs that may ultimately be earned based on management's probability assessment of the achievement of performance conditions at each reporting period. In addition, MPSUs are subject to the achievement of market conditions.