Note 3 - Stock-based Compensation - RSU Summary (Details) - $ / shares shares in Thousands		6 Months Ended	9 Months Ended
		Jun. 30, 2018	Sep. 30, 2018
Beginning balance (in shares)		4,144	4,144
Weighted-Average Grant Date Fair Value Per Share, beginning balance (in dollars per share)		$ 39.91	$ 39.91
Granted (in shares)			688
Weighted-Average Grant Date Fair Value Per Share, granted (in dollars per share)			$ 87.16
Vested (in shares)			(757)
Weighted-Average Grant Date Fair Value Per Share, vested (in dollars per share)			$ 43.69
Forfeited (in shares)			(20)
Weighted-Average Grant Date Fair Value Per Share, forfeited (in dollars per share)			$ 76.57
Ending balance (in shares)			4,055
Weighted-Average Grant Date Fair Value Per Share, ending balance (in dollars per share)			$ 47.04
Time-Based RSUs [Member]
Beginning balance (in shares)		258	258
Weighted-Average Grant Date Fair Value Per Share, beginning balance (in dollars per share)		$ 66.30	$ 66.30
Granted (in shares)			93
Weighted-Average Grant Date Fair Value Per Share, granted (in dollars per share)			$ 116.01
Vested (in shares)			(121)
Weighted-Average Grant Date Fair Value Per Share, vested (in dollars per share)			$ 59.01
Forfeited (in shares)			(14)
Weighted-Average Grant Date Fair Value Per Share, forfeited (in dollars per share)			$ 82.08
Ending balance (in shares)			216
Weighted-Average Grant Date Fair Value Per Share, ending balance (in dollars per share)			$ 90.79
PSUs and MPSUs [Member]
Beginning balance (in shares)		2,266	2,266
Weighted-Average Grant Date Fair Value Per Share, beginning balance (in dollars per share)		$ 48.59	$ 48.59
Granted (in shares)	[1]		595
Weighted-Average Grant Date Fair Value Per Share, granted (in dollars per share)			$ 82.64
Vested (in shares)			(636)
Weighted-Average Grant Date Fair Value Per Share, vested (in dollars per share)			$ 40.78
Forfeited (in shares)			(6)
Weighted-Average Grant Date Fair Value Per Share, forfeited (in dollars per share)			$ 63.16
Ending balance (in shares)			2,219
Weighted-Average Grant Date Fair Value Per Share, ending balance (in dollars per share)			$ 59.92
MSUs [Member]
Beginning balance (in shares)		1,620	1,620
Weighted-Average Grant Date Fair Value Per Share, beginning balance (in dollars per share)		$ 23.57	$ 23.57
Granted (in shares)	[1]
Weighted-Average Grant Date Fair Value Per Share, granted (in dollars per share)
Vested (in shares)
Weighted-Average Grant Date Fair Value Per Share, vested (in dollars per share)
Forfeited (in shares)
Weighted-Average Grant Date Fair Value Per Share, forfeited (in dollars per share)
Ending balance (in shares)		1,620
Weighted-Average Grant Date Fair Value Per Share, ending balance (in dollars per share)			$ 23.57

[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.