Summary of Significant Accounting Policies - Earnings Per Share (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Jan. 31, 2020	Oct. 31, 2019	Jul. 31, 2019	Apr. 30, 2019	Jan. 31, 2019	Oct. 31, 2018	Jul. 31, 2018	Apr. 30, 2018	Jan. 31, 2020	Jan. 31, 2019	Jan. 31, 2018
Accounting Policies [Abstract]
Net earnings for basic and diluted EPS	$ 201.2	$ 78.4	$ 136.3	$ 125.2	$ 204.5	$ 94.9	$ 144.7	$ 142.3	$ 541.1	$ 586.4	$ 370.1
Basic (shares)									121.1	122.9	124.5
Incremental shares based upon the assumed exercise of stock options and unvested restricted stock units									0.5	0.6	0.6
Weighted-average shares for diluted EPS									121.6	123.5	125.1
Antidilutive securities excluded from computation of earnings per share (shares)									1.3	0.7	0.6