Debt Schedule of Long-term Debt (Details) ¥ in Millions, $ in Millions		3 Months Ended
		Apr. 30, 2018 USD ($)	Apr. 30, 2018 JPY (¥)	Apr. 30, 2018 USD ($)	Jan. 31, 2018 USD ($)	Apr. 30, 2017 USD ($)
Debt Instrument [Line Items]
Long-term Debt				$ 891.7	$ 891.9	$ 889.9
Less unamortized discounts and debt issuance costs				8.8	9.0	9.4
Long-term debt				$ 882.9	882.9	880.5
2012 4.40% Series B Notes, due July 2042 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage			4.40%	4.40%
Long-term Debt	[1]			$ 250.0	250.0	250.0
Debt Instrument, Periodic Payment, Principal		$ 50.0
Debt Instrument, Face Amount				$ 250.0
2014 3.80% Senior Notes, due October 2024 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage			3.80%	3.80%
Long-term Debt	[2],[3]			$ 250.0	250.0	250.0
2014 4.90% Senior Notes, due October 2044 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage			4.90%	4.90%
Long-term Debt	[2],[3]			$ 300.0	300.0	300.0
2016 0.78% Senior Notes, due August 2026 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage			0.78%	0.78%
Long-term Debt	[2],[4]			$ 91.7	$ 91.9	$ 89.9
Debt Instrument, Face Amount | ¥			¥ 10,000.0

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

[2] b These agreements require lump sum repayments upon maturity.

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

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