Financial Instruments (Narrative) (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jul. 31, 2018	Jul. 31, 2017	Jul. 31, 2018	Jul. 31, 2017	Jan. 31, 2018
Investments, All Other Investments [Abstract]
Transfer from Level 1 to Level 2	$ 245,200,000		$ 245,200,000
Transfer from Level 2 to Level 1	122,900,000		122,900,000
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			146,600,000	$ 531,100,000
Equity securities	113,100,000		113,100,000		$ 112,300,000
Positive adjustment on certain of its privately held investments			6,200,000
Other than temporary impairment losses, investments	$ 2,800,000	$ 3,600,000	$ 4,800,000	$ 4,100,000