Debt - Senior Notes (Details) ¥ in Millions, $ in Millions		9 Months Ended
		Oct. 31, 2017 USD ($)	Oct. 31, 2017 JPY (¥)	Oct. 31, 2017 USD ($)	Jan. 31, 2017 USD ($)	Oct. 31, 2016 USD ($)
Debt Instrument [Line Items]
Long-term Debt, Gross				$ 888.3	$ 888.0	$ 895.4
2012 4.40% Series B Notes, due July 2042 [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal		$ 50.0
Debt Instrument, Face Amount				250.0
Long-term Debt, Gross	[1]			$ 250.0	250.0	250.0
2016 0.78% Senior Notes, due August 2026 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage			0.78%	0.78%
Debt Instrument, Face Amount | ¥			¥ 10,000.0
Long-term Debt, Gross	[2],[3]			$ 88.3	$ 88.0	$ 95.4
2010 1.72% Notes, due September 2016 [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage			1.72%	1.72%
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] d These Senior Notes were issued, at par, ¥10.0 billion.