Taxes - Additional Information (Detail) - USD ($)	1 Months Ended	3 Months Ended			6 Months Ended		12 Months Ended
	Jul. 31, 2017	Apr. 30, 2018	Jan. 31, 2018	Apr. 30, 2017	Apr. 30, 2018	Apr. 30, 2017	Oct. 31, 2018	Oct. 31, 2017	Jan. 31, 2017	Nov. 01, 2016
Taxes [Line Items]
Increase in income tax expense as result of the Tax Cuts and Jobs Act			$ 119,000,000
Provisional tax charge as result of deferred tax remeasurement due to Tax Cuts and Jobs Act			46,000,000
Provisional transition tax expense as result of the Tax Cuts and Jobs Act			71,000,000
Income taxes payable on foreign earnings as result of the Tax Cuts and Jobs Act		$ 18,000,000	78,000,000
Deferred income taxes		138,066,000			$ 138,066,000			$ 243,989,000
Share-based compensation, excess tax benefit, amount		3,300,000		$ 8,300,000	10,800,000	$ 11,300,000
Minimum
Taxes [Line Items]
Estimated potential decrease in underlying unrecognized tax benefits		0			0
Maximum
Taxes [Line Items]
Estimated potential decrease in underlying unrecognized tax benefits		$ 13,000,000			$ 13,000,000
Accounting Standards Update 2016-09 | New Accounting Pronouncement, Early Adoption, Effect
Taxes [Line Items]
Deferred income taxes									$ 106,500,000
Retained Earnings | Accounting Standards Update 2016-09 | New Accounting Pronouncement, Early Adoption, Effect
Taxes [Line Items]
Cumulative effect adjustment										$ 106,500,000
State
Taxes [Line Items]
Provisional transition tax expense as result of the Tax Cuts and Jobs Act			$ 2,000,000
Synopsys Hungary | Foreign Tax Authority | Tax Year 2011 - Tax Year 2013 | Hungarian Tax Authority
Taxes [Line Items]
Estimate of tax assessment	$ 47,000,000
Estimate of additional penalties and interest	18,000,000
Synopsys Hungary | Foreign Tax Authority | Fiscal Year 2015 and Beyond | Hungarian Tax Authority
Taxes [Line Items]
Estimate of tax assessment	$ 18,000,000
Scenario, Forecast
Taxes [Line Items]
Statutory federal income tax rate							23.40%