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Customer Trust Fund - Investment of Customer Trust Fund (Detail) - USD ($) $ in Millions | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2018 | Dec. 31, 2017 | Dec. 31, 2016
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 | $ 1,941.9
Invested customer trust funds, Fair Value | 3,418.0 | 4,091.4 | 3,697.3
Trust receivables, Fair Value | 8.6 | 8.3 | 5.5
Total customer trust funds, Fair Value | 3,426.6 | 4,099.7 | 3,702.8
Money market securities, investments carried at cost and other cash equivalents, Amortized Cost | 1,697.9 | 2,309.3 | 1,941.9
Invested customer trust funds, Amortized Cost | 3,448.4 | 4,097.2 | 3,686.8
Trust receivables, Amortized Cost | 8.6 | 8.3 | 5.5
Total customer trust funds, Amortized Cost | 3,457.0 | 4,105.5 | 3,692.3
Amortized Cost | 1,750.5 | 1,787.9 | 1,744.9
Gross Unrealized Gain | 2.3 | 7.5 | 17.3
Gross Unrealized Loss | (32.7) | (13.3) | (6.8)
Fair value | 1,720.1 | 1,782.1 | 1,755.4
U.S. Government and Agencies Securities [Member] |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized Cost | 548.6 | 584.6 | 607.7
Gross Unrealized Gain |  | 0.1 | 0.9
Gross Unrealized Loss | (19.1) | (7.1) | (4.7)
Fair value | 529.5 | 577.6 | 603.9
Canadian and Provincial Government Securities [Member] |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized Cost | 414.6 | 418.2 | 380.0
Gross Unrealized Gain | 2.1 | 6.6 | 12.9
Gross Unrealized Loss | (2.7) | (1.5) | (0.1)
Fair value | 414.0 | 423.3 | 392.8
Corporate Debt Securities [Member] |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized Cost | 513.1 | 472.3 | 511.7
Gross Unrealized Gain | 0.2 | 0.8 | 3.1
Gross Unrealized Loss | (6.8) | (2.5) | (1.2)
Fair value | 506.5 | 470.6 | 513.6
Asset-backed Securities [Member] |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized Cost | 249.6 | 280.8 | 192.2
Gross Unrealized Gain |  |  | 0.4
Gross Unrealized Loss | (3.6) | (1.8) | (0.3)
Fair value | 246.0 | 279.0 | 192.3
Mortgage-backed Securities [Member] |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized Cost | 9.7 | 15.0 | 28.7
Gross Unrealized Loss | (0.3) | (0.2) | (0.2)
Fair value | 9.4 | 14.8 | 28.5
Other Securities [Member] |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Amortized Cost | 14.9 | 17.0 | 24.6
Gross Unrealized Loss | (0.2) | (0.2) | (0.3)
Fair value | $ 14.7 | $ 16.8 | $ 24.3

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