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Income Taxes (Narrative) (Detail) - USD ($)	12 Months Ended
	Jan. 31, 2019	Jan. 31, 2018	Jan. 31, 2017	Jan. 31, 2016
Tax Credit Carryforward [Line Items]
Federal statutory rate	21.00%	33.80%	35.00%
Valuation allowance	$ 1,515,945,000	$ 625,030,000
Increase of valuation allowance on net deferred tax assets	891,000,000	333,000,000
Unrecognized tax benefits, accrued penalties and interest	0	0
Unrecognized tax benefits	130,771,000	$ 107,849,000	$ 116,801,000	$ 98,460,000
Unrecognized tax benefits that would impact effective tax rate	2,000,000
Federal
Tax Credit Carryforward [Line Items]
Net operating loss carryforwards	2,800,000,000
Federal | Research Tax Credit Carryforward
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	128,000,000
State
Tax Credit Carryforward [Line Items]
Net operating loss carryforwards	1,800,000,000
Foreign
Tax Credit Carryforward [Line Items]
Net operating loss carryforwards	703,000,000
California | State | Research Tax Credit Carryforward
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	$ 123,000,000

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Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.

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