Business Combinations - Schedule of Major Classes of Assets and Liabilities Allocated to Purchase Price (Detail) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Business Acquisition [Line Items]
Goodwill	$ 2,311.5	$ 2,031.8	$ 1,973.5
Ascender HCM Pty Ltd. [Member]
Business Acquisition [Line Items]
Cash and equivalents	3.1
Restricted cash	2.0
Trade receivables, prepaid expenses, and other current assets	15.8
Customer funds	16.2
Property, plant, and equipment and other assets	20.2
Goodwill	275.7
Other intangible assets, net	117.5
Accounts payable and other current liabilities	(30.5)
Customer funds obligations	(16.1)
Other non-current liabilities	(44.3)
Total purchase price	$ 359.6