Employee Benefit Plans - Reconciliation of Benefit Obligations, Plan Assets and Funded Status (Details) - USD ($) $ in Millions	12 Months Ended
	Jan. 31, 2020	Jan. 31, 2019	Jan. 31, 2018
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	$ 549.7
Fair value of plan assets at end of year	668.7	$ 549.7
Pension Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Projected benefit obligation at beginning of year	795.0	795.6
Service cost	17.0	17.9	$ 17.3
Interest cost	32.5	30.7	32.0
Participants' contributions	0.0	0.0
MMA retiree drug subsidy	0.0	0.0
Actuarial loss (gain)	139.8	(22.4)
Benefits paid	(28.2)	(26.8)
Projected benefit obligation at end of year	956.1	795.0	795.6
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	549.7	578.1
Actual return on plan assets	109.9	(20.1)
Employer contributions	37.3	18.5
Participants' contributions	0.0	0.0
MMA retiree drug subsidy	0.0	0.0
Benefits paid	(28.2)	(26.8)
Fair value of plan assets at end of year	668.7	549.7	578.1
Funded status at end of year	(287.4)	(245.3)
Other Postretirement Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Projected benefit obligation at beginning of year	76.1	78.5
Service cost	2.6	3.0	2.8
Interest cost	3.3	3.0	3.0
Participants' contributions	1.3	1.3
MMA retiree drug subsidy	0.1	0.1
Actuarial loss (gain)	15.4	(7.0)
Benefits paid	(2.7)	(2.8)
Projected benefit obligation at end of year	96.1	76.1	78.5
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0.0	0.0
Actual return on plan assets	0.0	0.0
Employer contributions	1.3	1.4
Participants' contributions	1.3	1.3
MMA retiree drug subsidy	0.1	0.1
Benefits paid	(2.7)	(2.8)
Fair value of plan assets at end of year	0.0	0.0	$ 0.0
Funded status at end of year	$ (96.1)	$ (76.1)