Commitments and Contingencies - Other (Details) $ in Millions, $ in Millions	3 Months Ended		12 Months Ended
	Apr. 30, 2020 USD ($)	Oct. 31, 2018 USD ($)	Jan. 31, 2019 USD ($)	Jan. 31, 2019 AUD ($)
Commitments and Contingencies Disclosure [Abstract]
Carrying value of precious scrap metals, net of expected recoveries	$ 12.3	$ 8.5
Compulsory acquisition offer				$ 48.0
Proceeds from advances, percentage of total compensation (percent)			90.00%	90.00%
Proceeds from advances			$ 31.1
Proceeds from final payments	50.5
Gain on acquisition	$ 31.4