Financial Instruments (Narrative) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Oct. 31, 2018	Oct. 31, 2017	Oct. 31, 2018	Oct. 31, 2017	Jan. 31, 2018
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
Proceeds from sale and maturity of marketable securities			256,200,000	$ 729,300,000
Equity securities	112,300,000		112,300,000		$ 112,300,000
Positive adjustment on certain of its privately held investments			6,200,000
Other than temporary impairment losses, investments	$ 0	$ 3,900,000	$ 4,800,000	$ 8,000,000.0