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Customer Trust Fund - Additional Information (Detail) - USD ($) $ in Millions | 9 Months Ended | 12 Months Ended
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Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Investment income from invested customer trust fund included in revenue | $ 49.6 | $ 34.2 | $ 46.5 | $ 39.1 | $ 36.9
Maturities ranging from one to 120 days [Member] |  |  |  |  |
Average customer trust funds held in liquidity portfolios, Percentage | 46.00% |  | 46.00% |  |
Maturities ranging from 120 days to 10 years [Member] |  |  |  |  |
Average customer trust funds held in fixed income portfolios, Percentage | 54.00% |  | 54.00% |  |

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- Definition Average customer trust funds maintained in fixed income portfolios percentage. + References No definition available. + Details
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- Definition Average customer trust funds maintained in liquidity portfolios percentage. + References No definition available. + Details
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- Definition Investment Income From Invested Customer Trust Fund + References No definition available. + Details
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- Details
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- Details
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