Summary of Significant Accounting Policies - Revenue Recognition (Details) - USD ($) $ 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
Disaggregation of Revenue [Line Items]
Net sales	$ 1,357.8	$ 1,014.6	$ 1,048.5	$ 1,003.1	$ 1,320.6	$ 1,012.4	$ 1,075.9	$ 1,033.2	$ 4,424.0	$ 4,442.1	$ 4,169.8
Jewelry Collections
Disaggregation of Revenue [Line Items]
Net sales									2,420.2	2,374.3	2,146.6
Engagement Jewelry
Disaggregation of Revenue [Line Items]
Net sales									1,139.5	1,157.4	1,111.9
Designer Jewelry
Disaggregation of Revenue [Line Items]
Net sales									514.1	544.5	551.2
Other products/services
Disaggregation of Revenue [Line Items]
Net sales									$ 350.2	$ 365.9	$ 360.1