Customer Funds - Investment of Customer Funds (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Money market securities, investments carried at cost and other cash equivalents, Fair Value	$ 2,105.0	$ 2,027.1
Invested customer funds, Fair Value	4,268.0	3,746.2
Receivables, Fair Value	16.0	13.2
Total customer funds, Fair Value	4,284.0	3,759.4
Money market securities, investments carried at cost and other cash equivalents, Amortized Cost	2,105.0	2,027.1
Invested customer funds, Amortized Cost	4,221.3	3,684.6
Receivables, Amortized Cost	16.0	13.2
Total customer funds, Amortized Cost	4,237.3	3,697.8
Amortized Cost	2,116.3	1,657.5
Gross Unrealized Gain	49.6	61.7
Gross Unrealized Loss	(2.9)	(0.1)
Fair value	2,163.0	1,719.1
U.S. Government and Agencies Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	541.9	494.0
Gross Unrealized Gain	17.7	21.6
Gross Unrealized Loss	(2.3)	(0.1)
Fair value	557.3	515.5
Canadian and Provincial Government Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	400.2	396.4
Gross Unrealized Gain	12.4	15.5
Gross Unrealized Loss	(0.1)
Fair value	412.5	411.9
Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	420.3	548.5
Gross Unrealized Gain	15.5	19.4
Gross Unrealized Loss	(0.3)
Fair value	435.5	567.9
Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	192.7	192.2
Gross Unrealized Gain	3.9	4.9
Fair value	196.6	197.1
Mortgage-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	7.4	9.9
Gross Unrealized Gain	0.1	0.2
Fair value	7.5	10.1
Other Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	44.5	16.5
Gross Unrealized Gain		0.1
Gross Unrealized Loss	(0.2)
Fair value	44.3	$ 16.6
Other Short-Term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	509.3
Fair value	$ 509.3

Funds held for clients amortized cost.

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Invested customer funds amortized cost.

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Invested customer funds fair value.

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Money market securities investments carried at cost and other cash equivalents fair value.

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Receivables amortized cost.

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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Amount of unrealized gain before deducting unrealized loss on investments in debt and equity securities classified as available-for-sale securities.

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Amount of unrealized loss before deducting unrealized gain on investments in available-for-sale securities.

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Carrying amount as of the balance sheet date of the funds held on behalf of others and that are expected to be liquidated within one year or the normal operating cycle, if longer. This does not include funds held under reinsurance agreements.

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Investment in short-term money-market instruments (such as commercial paper, banker's acceptances, repurchase agreements, government securities, certificates of deposit, and so forth) which are highly liquid (that is, readily convertible to known amounts of cash) and so near their maturity that they present an insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify as cash equivalents by definition. Original maturity means an original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months.

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Fair value portion of receivables, including, but not limited to, trade account receivables, note receivables, and loan receivables.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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