Debt - Schedule of Long-term Debt (Details) ¥ in Billions			3 Months Ended
		Oct. 25, 2018 USD ($)	Apr. 30, 2020 USD ($)	Apr. 30, 2020 JPY (¥)	Apr. 30, 2020 USD ($)	Jan. 31, 2020 USD ($)	Apr. 30, 2019 USD ($)
Debt Instrument [Line Items]
Long-term debt					$ 893,800,000	$ 891,900,000	$ 889,500,000
Less: unamortized discounts and debt issuance costs					(7,700,000)	(7,800,000)	(8,300,000)
Long-term debt					$ 886,100,000	884,100,000	881,200,000
Debt covenant compliance			in compliance
2012 4.40% Series B Notes, due July 2042
Debt Instrument [Line Items]
Debt instrument stated rate (percent)				4.40%	4.40%
Long-term debt	[1]				$ 250,000,000.0	250,000,000.0	250,000,000.0
Debt instrument periodic principal payment			$ 50,000,000.0
2014 3.80% Senior Notes, due October 2024
Debt Instrument [Line Items]
Debt instrument stated rate (percent)				3.80%	3.80%
Long-term debt	[2],[3]				$ 250,000,000.0	250,000,000.0	250,000,000.0
2014 4.90% Senior Notes, due October 2044
Debt Instrument [Line Items]
Debt instrument stated rate (percent)				4.90%	4.90%
Long-term debt	[2],[3]				$ 300,000,000.0	300,000,000.0	300,000,000.0
2016 0.78% Senior Notes, due August 2026
Debt Instrument [Line Items]
Debt instrument stated rate (percent)				0.78%	0.78%
Long-term debt	[3],[4]				$ 93,800,000	$ 91,900,000	$ 89,500,000
Debt instrument face amount | ¥				¥ 10.0
Revolving Credit Facility
Debt Instrument [Line Items]
Amount of commitment		$ 750,000,000.0
Revolving Credit Facility | Line of Credit
Debt Instrument [Line Items]
Debt term		5 years
Drawings			$ 500,000,000.0

[1] The agreements governing these Senior Notes require repayments of $50.0 million in aggregate every five years beginning in July 2022.

[2] These Senior Notes were issued at a discount, which will be amortized until the debt maturity.

[3] These agreements require lump sum repayments upon maturity.

[4] These Senior Notes were issued at par, ¥10.0 billion.