Business combination (Preliminary Allocation of Consideration) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended		14 Months Ended
	Oct. 31, 2020	Oct. 31, 2020	Oct. 31, 2019	Oct. 31, 2020	Jan. 31, 2020	Aug. 30, 2019
Business Acquisition [Line Items]
Goodwill	$ 1,326,793	$ 1,326,793		$ 1,326,793	$ 1,332,631
Goodwill, adjustments	7,000	5,838	$ 0
Wage Works Inc.
Business Acquisition [Line Items]
Cash and cash equivalents	392,300	392,300		392,300		$ 406,800
Cash and cash equivalents, adjustments				(14,500)
Other current assets	59,000	59,000		59,000		56,500
Other current assets, adjustments				2,500
Property, plant, and equipment	26,600	26,600		26,600		26,600
Operating lease right-of-use assets	42,500	42,500		42,500		42,500
Intangible assets	715,300	715,300		715,300		715,300
Goodwill	1,322,100	1,322,100		1,322,100		1,330,500
Goodwill, adjustments				(8,400)
Other assets	5,900	5,900		5,900		5,900
Client-held funds obligation	(220,300)	(220,300)		(220,300)		(237,500)
Client-held funds obligation, adjustments				17,200
Other current liabilities	(72,400)	(72,400)		(72,400)		(69,100)
Other current liabilities, adjustments				(3,300)
Other long-term liabilities	(26,700)	(26,700)		(26,700)		(26,700)
Deferred tax liability	(122,200)	(122,200)		(122,200)		(128,700)
Deferred tax liability, adjustments				6,500
Total consideration paid	$ 2,122,100	$ 2,122,100		2,122,100		$ 2,122,100
Total consideration paid, adjustments				$ 0