Debt - Schedule of Long-term Debt (Details) $ in Millions, ¥ in Billions		6 Months Ended
		Jul. 31, 2019 USD ($)	Jul. 31, 2019 JPY (¥)	Jul. 31, 2019 USD ($)	Jan. 31, 2019 USD ($)	Jul. 31, 2018 USD ($)
Debt Instrument [Line Items]
Long-term debt				$ 892.1	$ 891.8	$ 890.1
Less: unamortized discounts and debt issuance costs				(8.1)	(8.4)	(8.7)
Long-term debt				$ 884.0	883.4	881.4
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]			$ 250.0	250.0	250.0
Debt instrument periodic principal payment		$ 50.0
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]			$ 250.0	250.0	250.0
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]			$ 300.0	300.0	300.0
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]			$ 92.1	$ 91.8	$ 90.1
Debt instrument face amount | ¥			¥ 10.0

[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.