Financial Instruments (Narrative) (Details) - USD ($)	3 Months Ended
	Apr. 30, 2018	Apr. 30, 2017	Jan. 31, 2018
Investments, All Other Investments [Abstract]
Transfer from Level 1 to Level 2	$ 287,200,000
Transfer from Level 2 to Level 1	175,200,000
Securities in a continuous unrealized loss position for more than 12 months	0		$ 0
Gain (loss) from sale or redemption of available for sale securities	0
Proceeds from sale and maturity of marketable securities	74,800,000	$ 382,600,000
Equity securities	114,800,000		$ 112,300,000
Other than temporary impairment losses, investments	2,000,000.0	$ 500,000
Positive adjustment on certain of its privately held investments	$ 4,600,000