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Cash, Cash Equivalents and Marektable Securities (Details) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
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Cash and Cash Equivalents [Line Items] |  |
Money Market Funds, Overnight Holdings, at Carrying Value | $ 503,221 | $ 349,857
Short-term Investments | 82,521 | 47,946
Cash | 33,692 | 36,521
Drafts Payable | (30,059) | (32,254)
Cash, Cash Equivalents, and Short-term Investments | 589,375 | 402,070
Client Prepayments Included in Cash Balance | 35,328 | 59,612
Withholding associated with federal and state income taxes, employment taxes and other payroll deductions included in cash balance | 342,000 | 234,600
Client Prepayments Included in Cash Balance | (35,328) | (59,612)
Money Market Funds [Member] |  |
Cash and Cash Equivalents [Line Items] |  |
Money Market Funds, Overnight Holdings, at Carrying Value | 0 | 0
Short-term Investments | 34,529 | 34,728
Cash | 0 | 0
Drafts Payable | 0 | 0
Cash, Cash Equivalents, and Short-term Investments | 34,529 | 34,728
Cash and Cash Equivalents [Member] |  |
Cash and Cash Equivalents [Line Items] |  |
Money Market Funds, Overnight Holdings, at Carrying Value | 503,221 | 349,857
Short-term Investments | 47,992 | 13,218
Cash | 33,692 | 36,521
Drafts Payable | (30,059) | (32,254)
Cash, Cash Equivalents, and Short-term Investments | $ 554,846 | $ 367,342

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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 draft payable by broker-dealer. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher SEC -Name Rule 15c3-1 -Number 240 -Section 15c3-1 + 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://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 210 -Section 45 -Paragraph 20 -URI http://asc.fasb.org/extlink&oid=118262064&loc=SL116631418-115840 Reference 2: 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=120391452&loc=d3e13212-122682 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 210 -Section 45 -Paragraph 21 -URI http://asc.fasb.org/extlink&oid=118262064&loc=SL116631419-115840 + 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=120391452&loc=d3e13212-122682 + Details
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- Definition Amount of investments including trading securities, available-for-sale securities, held-to-maturity securities, and short-term investments classified as other and current. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(4)) -URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(5)) -URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(1)(g)) -URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910 Reference 4: 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(2)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
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