Financial Instruments (Narrative) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Oct. 31, 2019	Oct. 31, 2018	Oct. 31, 2019	Oct. 31, 2018	Jan. 31, 2019
Investments, All Other Investments [Abstract]
Securities in a continuous unrealized loss position for more than 12 months	$ 0		$ 0		$ 0
Gain (loss) from sale or redemption of available for sale securities			0	$ 0
Proceeds from sale and maturity of marketable securities	0	$ 109,600,000	27,400,000	256,200,000
Direct investments in privately held companies	121,800,000		121,800,000		$ 111,600,000
Positive adjustment on certain of its privately held investments			2,500,000	6,200,000
Cumulative adjustment on certain of its privately held investments	8,700,000		8,700,000
Other than temporary impairment losses, investments			4,300,000	$ 4,800,000
Cumulative impairments and negative adjustments on certain of its privately held investments	$ 9,000,000.0		$ 9,000,000.0