Customer Trust Fund - Investment of Customer Trust Fund (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Money market securities, investments carried at cost and other cash equivalents, Fair Value	$ 1,697.9	$ 2,309.3
Invested customer trust funds, Fair Value	3,418.0	4,091.4
Trust receivables, Fair Value	8.6	8.3
Total customer trust funds, Fair Value	3,426.6	4,099.7
Money market securities, investments carried at cost and other cash equivalents, Amortized Cost	1,697.9	2,309.3
Invested customer trust funds, Amortized Cost	3,448.4	4,097.2
Trust receivables, Amortized Cost	8.6	8.3
Total customer trust funds, Amortized Cost	3,457.0	4,105.5
Amortized Cost	1,750.5	1,787.9
Gross Unrealized Gain	2.3	7.5
Gross Unrealized Loss	(32.7)	(13.3)
Fair value	1,720.1	1,782.1
U.S. Government and Agencies Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	548.6	584.6
Gross Unrealized Gain		0.1
Gross Unrealized Loss	(19.1)	(7.1)
Fair value	529.5	577.6
Canadian and Provincial Government Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	414.6	418.2
Gross Unrealized Gain	2.1	6.6
Gross Unrealized Loss	(2.7)	(1.5)
Fair value	414.0	423.3
Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	513.1	472.3
Gross Unrealized Gain	0.2	0.8
Gross Unrealized Loss	(6.8)	(2.5)
Fair value	506.5	470.6
Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	249.6	280.8
Gross Unrealized Loss	(3.6)	(1.8)
Fair value	246.0	279.0
Mortgage-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	9.7	15.0
Gross Unrealized Loss	(0.3)	(0.2)
Fair value	9.4	14.8
Other Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	14.9	17.0
Gross Unrealized Loss	(0.2)	(0.2)
Fair value	$ 14.7	$ 16.8

Funds held for clients amortized cost.

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- Definition

Invested customer trust funds.

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Invested customer trust funds amortized cost.

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

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Trust receivables.

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Trust receivables amortized cost.

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- Definition

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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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-Topic 320
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-Section 50
-Paragraph 2
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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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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
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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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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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