Financial Instruments - Narrative (Detail) - USD ($)	12 Months Ended
	Jan. 31, 2019	Jan. 31, 2018	Jan. 31, 2017
Investments, All Other Investments [Abstract]
Securities in continuous unrealized loss position for greater than twelve months	$ 0	$ 0
Available-for-sale securities gross gains (losses)	(1,300,000)	(300,000)	$ 1,500,000
Proceeds from sale and maturity of marketable securities	531,000,000.0	1,080,000,000.00
Non-marketable equity securities investments without readily determinable fair value	111,600,000	112,300,000
Positive adjustment on privately held investments	6,200,000
Other than temporary impairment losses, investments	$ 4,800,000	$ 15,500,000	$ 1,300,000