Note 17 - Income Taxes - Percentage of Income Before Income Taxes (Details)		12 Months Ended
		Aug. 31, 2019	Dec. 31, 2018	Aug. 31, 2018		Dec. 31, 2017	Aug. 31, 2017
Tax at U.S. Federal statutory tax rate		21.00%	21.00%	25.70%		35.00%	35.00%
Increase (decrease) in taxes resulting from:
State and local taxes, net of U.S. federal income tax benefit		4.00%		2.90%			1.80%
Foreign income at other than U.S. rates		(1.40%)		(3.20%)			(7.00%)	[1]
Foreign derived intangible income deduction		(1.70%)		0.00%			0.00%
Domestic production activities deduction		0.00%		(1.60%)			(2.10%)
Income tax benefits from R&D tax credits		(3.50%)		(3.70%)			(3.30%)
Income tax benefits from foreign tax credits		0.00%		0.00%			(0.30%)
Share-based payments(1)	[2]	(3.20%)		(2.70%)			0.00%
One-time transition tax from TCJA		(0.40%)		6.60%	[3]		0.00%
Other, net		1.60%		0.10%			0.90%
Effective tax rate		16.40%		24.10%			25.00%

[1] Includes a 200 basis point benefit as a result of FactSet's global realignment. Effective September 1, 2016, FactSet realigned certain aspects of its global operations from FactSet Research Systems Inc., its U.S. parent company, to FactSet UK Limited, a U.K. operating company, to better position the Company to serve its growing client base outside the U.S. This realignment allows the Company to further implement strategic corporate objectives and helps achieve operational and financial efficiencies, while complementing FactSet's increasing global growth and reach.

[2] During the first quarter of fiscal 2018, FactSet adopted an accounting standard that requires all excess tax benefits or deficiencies related to share-based payments to be reported within the consolidated statement of income that were previously reported within equity. The adoption of this standard resulted in the recognition of $9.5 million of excess tax benefits to FactSet's provision for income taxes during fiscal 2018.

[3] The enactment of the TCJA resulted in a one-time transition tax expense of $23.2 million during the second quarter of fiscal 2018.