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Cash, Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Cash and Cash Equivalents [Line Items] |  |
Overnight Holdings | $ 270,172 | $ 338,112
Short-term Investments | 81,565 | 24,594
Cash | 29,585 | 26,700
Drafts Payable | (15,447) | (33,186)
Cash, Cash Equivalents, and Short-term Investments | 365,875 | 356,220
Payroll Withholdings Included in Cash Balance | 181,300 | 271,500
Client Prepayments Included in Cash Balance | 18,000 | 23,600
Money Market Funds [Member] |  |
Cash and Cash Equivalents [Line Items] |  |
Overnight Holdings | 0 | 0
Short-term Investments | 37,576 | 1,960
Cash | 0 | 0
Drafts Payable | 0 | 0
Cash, Cash Equivalents, and Short-term Investments | 37,576 | 1,960
Cash and Cash Equivalents [Member] |  |
Cash and Cash Equivalents [Line Items] |  |
Overnight Holdings | 270,172 | 338,112
Short-term Investments | 43,989 | 22,634
Cash | 29,585 | 26,700
Drafts Payable | (15,447) | (33,186)
Cash, Cash Equivalents, and Short-term Investments | $ 328,299 | $ 354,260

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- Definition Client prepayments included in the cash balance. + References No definition available. + Details
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- Definition 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, these investments are only held overnight. + References No definition available. + Details
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- Definition Withholdings associated with liabilities related to taxes and payroll deductions which is included in the cash balance. + References No definition available. + Details
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- Definition Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Excludes cash and cash equivalents within disposal group and discontinued operation. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.1) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition 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. + References No definition available. + Details
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- Definition Cash includes currency on hand as well as demand deposits with banks or financial institutions. It also includes other kinds of accounts that have the general characteristics of demand deposits in that the customer may deposit additional funds at any time and effectively may withdraw funds at any time without prior notice or penalty. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid Investments that are both readily convertible to known amounts of cash, and so near their maturity that they present minimal risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means 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. Short-term investments, exclusive of cash equivalents, generally consist of marketable securities intended to be sold within one year (or the normal operating cycle if longer) and may include trading securities, available-for-sale securities, or held-to-maturity securities (if maturing within one year), as applicable. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.1) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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