Summary of Significant Accounting Policies (Policies)	3 Months Ended
Mar. 31, 2023
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying unaudited interim consolidated financial statements and notes have been prepared in accordance with generally accepted accounting principles in the United States of America (“U.S. GAAP”) and applicable rules and regulations of the SEC regarding interim financial statements that permit reduced disclosure for interim periods. In the opinion of management, the unaudited consolidated financial statements reflect all adjustments of a normal recurring nature that are necessary for a fair presentation of the results for the interim periods presented. These unaudited consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements and related notes presented in the Form 10-K. The results of operations for the three months ended March 31, 2023 are not necessarily indicative of the results expected for the full year.
Recently Adopted / Issued Accounting Pronouncements	Recently Adopted Accounting Pronouncements In March 2020, the Financial Accounting Standards Board (“FASB”) issued ASU No. 2020-04, “Reference Rate Reform (Topic 848) Facilitation of the Effects of Reference Rate Reform on Financial Reporting” (“ASU 2020-04”). ASU 2020-04 provides temporary optional expedients and exceptions for applying U.S. GAAP to contracts, hedging relationships and other transactions affected by reference rate reform. Prior to August 24, 2022, our floating-to-fixed interest rate swap was outstanding to offset the rate variability associated with our outstanding indebtedness. As discussed in Note 7, the interest rate swap was terminated on August 24, 2022. As a result, the adoption of ASU 2020-04 had no material impact on our unaudited interim consolidated financial statements. In January 2021, the FASB issued ASU No. 2021-01, “Reference Rate Reform (Topic 848) Scope” (“ASU 2021-01”), which clarifies that certain optional expedients and exceptions in Topic 848 for contract modifications and hedge accounting apply to derivative instruments that are affected by the discounting transition. ASU 2021-01 amends the expedients and exceptions in Topic 848 to capture the incremental consequences of the scope clarification and to tailor the existing guidance to derivative instruments affected by the discounting transition. As discussed in Note 7, the interest rate swap was terminated on August 24, 2022. As a result, the adoption of ASU 2021-01 had no material impact on our unaudited interim consolidated financial statements.
Use of Estimates	Use of Estimates The preparation of consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. Significant estimates include income taxes, loss contingencies, the useful life of property and equipment and intangible assets, the life of our client relationships, the fair value of our stock-based awards and the fair value of our financial instruments, intangible assets and goodwill. These estimates are based on historical experience where applicable and other assumptions that management believes are reasonable under the circumstances. Actual results could materially differ from these estimates.
Seasonality	Seasonality Our revenues are seasonal in nature and generally we expect our first and fourth quarter recurring revenues to be higher than other quarters during the year. Recurring revenues include revenues relating to the annual processing of payroll tax filing forms and ACA form filing requirements, such as Form W-2, Form 1099, and Form 1095 and revenues from processing unscheduled payroll runs (such as bonuses) for our clients. As payroll tax forms are typically processed in the first quarter of the year, first quarter recurring revenues and margins are positively impacted. In addition, unscheduled payroll runs at the end of the year often result in increased recurring revenues in the fourth quarter. These seasonal fluctuations in revenues can also have an impact on gross profits. Historical results impacted by these seasonal trends should not be considered a reliable indicator of our future results of operations.
Funds Held for Clients and Client Funds Obligation	Funds Held for Clients and Client Funds Obligation As part of our payroll and tax filing application, we (i) collect client funds to satisfy their respective federal, state and local employment tax obligations, (ii) remit such funds to the appropriate taxing authorities and accounts designated by our clients, and (iii) manage client tax filings and any related correspondence with taxing authorities. Amounts collected by us from clients for their federal, state and local employment taxes are invested by us, and we earn interest on these funds during the interval between receipt and disbursement. These investments are shown in our consolidated balance sheets as funds held for clients, and the associated liability for the tax filings is shown as client funds obligation. The liability is recorded in the accompanying consolidated balance sheets at the time we obtain the funds from clients. The client funds obligation represents liabilities that will be repaid within one year of the consolidated balance sheet date. As of March 31, 2023 and December 31, 2022, the funds held for clients were invested in money market funds, demand deposit accounts, commercial paper and certificates of deposit. Additionally, as of March 31, 2023, the funds held for clients were invested in U.S. treasury securities with an original maturity of greater than one year. Short-term investments in commercial paper and certificates of deposit with an original maturity greater than three months are classified as available-for-sale securities, and are also included within the funds held for clients line item in the consolidated balance sheets. U.S. treasury securities with an original maturity of greater than one year are also classified as available-for-sale securities and included within the funds held for clients line item in the consolidated balance sheets. These available-for-sale securities are recorded in the consolidated balance sheets at fair value, with the difference between the amortized cost and fair value of these available-for-sale securities recorded as unrealized net gains (losses) on available-for-sale securities and are included within comprehensive earnings (loss) in the consolidated statements of comprehensive income. Funds held for clients are classified as a current asset in the consolidated balance sheets because the funds are held solely to satisfy the client funds obligation. Additionally, the funds held for clients is classified as restricted cash and restricted cash equivalents and presented within the reconciliation of cash, cash equivalents, restricted cash and restricted cash equivalents on the consolidated statements of cash flows.
Stock Repurchase Plan	Stock Repurchase Plan In May 2016, our Board of Directors authorized a stock repurchase plan allowing for the repurchase of shares of our common stock in open market transactions at prevailing market prices, in privately negotiated transactions or by other means in accordance with federal securities laws, including Rule 10b5-1 programs. Since the initial authorization of the stock repurchase plan, our Board of Directors has amended and extended and authorized new stock repurchase plans from time to time. Most recently, in August 2022, our Board of Directors authorized the repurchase of up to $1.1 billion of our common stock. As of March 31, 2023, there was $1.1 billion available for repurchases under our stock repurchase plan. Our stock repurchase plan may be suspended or discontinued at any time. The actual timing, number and value of shares repurchased depends on a number of factors, including the market price of our common stock, general market and economic conditions, shares withheld for taxes associated with the vesting of restricted stock and other corporate considerations. The current stock repurchase plan will expire on August 15, 2024. During the three months ended March 31, 2023, we repurchased an aggregate of 1,923 shares of our common stock at an average cost of $325.37 per share, all of which were shares withheld to satisfy tax withholding obligations for certain employees upon the vesting of restricted stock.

Summary of Significant Accounting Policies (Policies)

3 Months Ended

Mar. 31, 2023

The accompanying unaudited interim consolidated financial statements and notes have been prepared in accordance with generally accepted accounting principles in the United States of America (“U.S. GAAP”) and applicable rules and regulations of the SEC regarding interim financial statements that permit reduced disclosure for interim periods. In the opinion of management, the unaudited consolidated financial statements reflect all adjustments of a normal recurring nature that are necessary for a fair presentation of the results for the interim periods presented. These unaudited consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements and related notes presented in the Form 10-K. The results of operations for the three months ended March 31, 2023 are not necessarily indicative of the results expected for the full year.

Recently Adopted / Issued Accounting Pronouncements

Recently Adopted Accounting Pronouncements

In March 2020, the Financial Accounting Standards Board (“FASB”) issued ASU No. 2020-04, “Reference Rate Reform (Topic 848) Facilitation of the Effects of Reference Rate Reform on Financial Reporting” (“ASU 2020-04”). ASU 2020-04 provides temporary optional expedients and exceptions for applying U.S. GAAP to contracts, hedging relationships and other transactions affected by reference rate reform. Prior to August 24, 2022, our floating-to-fixed interest rate swap was outstanding to offset the rate variability associated with our outstanding indebtedness. As discussed in Note 7, the interest rate swap was terminated on August 24, 2022. As a result, the adoption of ASU 2020-04 had no material impact on our unaudited interim consolidated financial statements.

In January 2021, the FASB issued ASU No. 2021-01, “Reference Rate Reform (Topic 848) Scope” (“ASU 2021-01”), which clarifies that certain optional expedients and exceptions in Topic 848 for contract modifications and hedge accounting apply to derivative instruments that are affected by the discounting transition. ASU 2021-01 amends the expedients and exceptions in Topic 848 to capture the incremental consequences of the scope clarification and to tailor the existing guidance to derivative instruments affected by the discounting transition. As discussed in Note 7, the interest rate swap was terminated on August 24, 2022. As a result, the adoption of ASU 2021-01 had no material impact on our unaudited interim consolidated financial statements.

Use of Estimates

The preparation of consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the amounts reported in the financial statements and accompanying notes. Significant estimates include income taxes, loss contingencies, the useful life of property and equipment and intangible assets, the life of our client relationships, the fair value of our stock-based awards and the fair value of our financial instruments, intangible assets and goodwill. These estimates are based on historical experience where applicable and other assumptions that management believes are reasonable under the circumstances. Actual results could materially differ from these estimates.

Seasonality

Our revenues are seasonal in nature and generally we expect our first and fourth quarter recurring revenues to be higher than other quarters during the year. Recurring revenues include revenues relating to the annual processing of payroll tax filing forms and ACA form filing requirements, such as Form W-2, Form 1099, and Form 1095 and revenues from processing unscheduled payroll runs (such as bonuses) for our clients. As payroll tax forms are typically processed in the first quarter of the year, first quarter recurring revenues and margins are positively impacted. In addition, unscheduled payroll runs at the end of the year often result in increased recurring revenues in the fourth quarter. These seasonal fluctuations in revenues can also have an impact on gross profits. Historical results impacted by these seasonal trends should not be considered a reliable indicator of our future results of operations.

Funds Held for Clients and Client Funds Obligation

As part of our payroll and tax filing application, we (i) collect client funds to satisfy their respective federal, state and local employment tax obligations, (ii) remit such funds to the appropriate taxing authorities and accounts designated by our clients, and (iii) manage client tax filings and any related correspondence with taxing authorities. Amounts collected by us from clients for their federal, state and local employment taxes are invested by us, and we earn interest on these funds during the interval between receipt and disbursement.

These investments are shown in our consolidated balance sheets as funds held for clients, and the associated liability for the tax filings is shown as client funds obligation. The liability is recorded in the accompanying consolidated balance sheets at the time we obtain the funds from clients. The client funds obligation represents liabilities that will be repaid within one year of the consolidated balance sheet date. As of March 31, 2023 and December 31, 2022, the funds held for clients were invested in money market funds, demand deposit accounts, commercial paper and certificates of deposit. Additionally, as of March 31, 2023, the funds held for clients were invested in U.S. treasury securities with an original maturity of greater than one year. Short-term investments in commercial paper and certificates of deposit with an original maturity greater than three months are classified as available-for-sale securities, and are also included within the funds held for clients line item in the consolidated balance sheets. U.S. treasury securities with an original maturity of greater than one year are also classified as available-for-sale securities and included within the funds held for clients line item in the consolidated balance sheets. These available-for-sale securities are recorded in the consolidated balance sheets at fair value, with the difference between the amortized cost and fair value of these available-for-sale securities recorded as unrealized net gains (losses) on available-for-sale securities and are included within comprehensive earnings (loss) in the consolidated statements of comprehensive income. Funds held for clients are classified as a current asset in the consolidated balance sheets because the funds are held solely to satisfy the client funds obligation. Additionally, the funds held for clients is classified as restricted cash and restricted cash equivalents and presented within the reconciliation of cash, cash equivalents, restricted cash and restricted cash equivalents on the consolidated statements of cash flows.

Stock Repurchase Plan

In May 2016, our Board of Directors authorized a stock repurchase plan allowing for the repurchase of shares of our common stock in open market transactions at prevailing market prices, in privately negotiated transactions or by other means in accordance with federal securities laws, including Rule 10b5-1 programs. Since the initial authorization of the stock repurchase plan, our Board of Directors has amended and extended and authorized new stock repurchase plans from time to time. Most recently, in August 2022, our Board of Directors authorized the repurchase of up to $1.1 billion of our common stock. As of March 31, 2023, there was $1.1 billion available for repurchases under our stock repurchase plan. Our stock repurchase plan may be suspended or discontinued at any time. The actual timing, number and value of shares repurchased depends on a number of factors, including the market price of our common stock, general market and economic conditions, shares withheld for taxes associated with the vesting of restricted stock and other corporate considerations. The current stock repurchase plan will expire on August 15, 2024.

During the three months ended March 31, 2023, we repurchased an aggregate of 1,923 shares of our common stock at an average cost of $325.37 per share, all of which were shares withheld to satisfy tax withholding obligations for certain employees upon the vesting of restricted stock.