Summary of Significant Accounting Policies - Revenue Recognition (Details) - USD ($) $ 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
Disaggregation of Revenue [Line Items]
Net sales	$ 1,320.6	$ 1,012.4	$ 1,075.9	$ 1,033.2	$ 1,334.3	$ 976.2	$ 959.7	$ 899.6	$ 4,442.1	$ 4,169.8	$ 4,001.8
Jewelry Collections
Disaggregation of Revenue [Line Items]
Net sales									2,374.1	2,146.6	1,991.0
Engagement Jewelry
Disaggregation of Revenue [Line Items]
Net sales									1,157.4	1,111.9	1,174.9
Designer Jewelry
Disaggregation of Revenue [Line Items]
Net sales									544.5	551.2	529.1
Other products/services
Disaggregation of Revenue [Line Items]
Net sales									$ 366.1	$ 360.1	$ 306.8