Summary of Significant Accounting Policies - Earnings Per Share (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Jan. 31, 2019	Oct. 31, 2018	Jul. 31, 2018	Apr. 30, 2018	Jan. 31, 2018	Oct. 31, 2017	Jul. 31, 2017	Apr. 30, 2017	Jan. 31, 2019	Jan. 31, 2018	Jan. 31, 2017
Accounting Policies [Abstract]
Net earnings for basic and diluted EPS	$ 204.5	$ 94.9	$ 144.7	$ 142.3	$ 61.9	$ 100.2	$ 115.0	$ 92.9	$ 586.4	$ 370.1	$ 446.1
Basic (shares)									122.9	124.5	125.1
Incremental shares based upon the assumed exercise of stock options and unvested restricted stock units									0.6	0.6	0.4
Weighted-average shares for diluted EPS									123.5	125.1	125.5
Antidilutive securities excluded from computation of earnings per share (shares)									0.7	0.6	1.3