Summary of Significant Accounting Policies (Policies)	9 Months Ended
Summary of Significant Accounting Policies (Policies)	Sep. 30, 2017
Accounting Policies [Abstract]
Basis of presentation	Basis of Presentation — The accompanying interim condensed consolidated balance sheets as of September 30, 2017 and December 31, 2016, the condensed consolidated statements of operations for the three and nine months ended September 30, 2017 and 2016, the condensed consolidated statements of comprehensive income for the three and nine months ended September 30, 2017 and 2016, and the condensed consolidated statements of cash flows for the nine months ended September 30, 2017 and 2016 are unaudited and have been prepared in accordance with accounting principles generally accepted in the United States of America (“U.S. GAAP”) and applicable rules and regulations of the SEC for interim financial reporting. Accordingly, certain information and footnote disclosures have been condensed or omitted pursuant to SEC rules that would ordinarily be required by U.S. GAAP for complete financial statements. These unaudited interim condensed consolidated financial statements should be read in conjunction with the audited consolidated financial statements and accompanying notes thereto for the fiscal year ended December 31, 2016 included in the Company’s Annual Report on Form 10-K filed with the SEC on February 28, 2017. The unaudited interim condensed consolidated financial statements have been prepared on the same basis as the audited consolidated financial statements and, in the opinion of management, reflect all adjustments consisting of normal recurring accruals considered necessary to present fairly the Company’s financial position as of September 30, 2017, results of its operations for the three and nine months ended September 30, 2017 and 2016, comprehensive income for the three and nine months ended September 30, 2017 and 2016, and cash flows for the nine months ended September 30, 2017 and 2016. The results of operations for the three and nine months ended September 30, 2017 are not necessarily indicative of the results that may be expected for the year ending December 31, 2017.
Income taxes	Income Taxes — The Company’s interim period provision for income taxes is computed by using an estimate of the annual effective tax rate, adjusted for discrete items taken into account in the relevant period, if any. Each quarter, the annual effective income tax rate is recomputed and if there are material changes in the estimate, a cumulative adjustment is made.
Accounts receivable	Accounts Receivable — Accounts receivable are recorded at original invoice amount less an allowance that management believes will be adequate to absorb estimated losses on uncollectible accounts. This allowance is based on an evaluation of the collectability of accounts receivable and prior bad debt experience. Accounts receivable are written off when deemed uncollectible. Unbilled receivables consist of revenue recognized in excess of billings, substantially all of which is expected to be billed and collected within one year. As of September 30, 2017 and December 31, 2016, unbilled accounts receivable of $14.4 million and $14.1 million, respectively, were included in accounts receivable on the Company's consolidated balance sheets. In general, there is a direct relationship between the Company's accounts receivable balance and its transaction volume.
Recently issued accounting pronouncements	Recently Adopted Accounting Pronouncements — In November 2015, the Financial Accounting Standards Board ("FASB") issued Accounting Standards Update ("ASU") No. 2015-17, Income Taxes: Balance Sheet Classification of Deferred Taxes, which simplifies the presentation of deferred income taxes by requiring that all deferred tax assets and liabilities be classified as noncurrent in the consolidated balance sheets. ASU No. 2015-17 is effective for annual reporting periods beginning after December 15, 2016 and interim periods within those annual periods, and may be applied either prospectively or retrospectively to all periods presented. The Company adopted ASU No. 2015-17 prospectively on January 1, 2017, and the adoption did not have a material impact on its consolidated financial statements, aside from a balance sheet reclassification from short-term to long-term deferred income taxes. In October 2016, the FASB issued ASU No. 2016-16, Income Taxes: Intra-Entity Transfers of Assets Other Than Inventory, which requires entities to recognize the income tax consequences of intra-entity transfers of assets other than inventory when those transfers occur. ASU No. 2016-16 is effective for annual reporting periods beginning after December 15, 2017, including interim periods within those annual periods. The Company early adopted ASU No. 2016-16 on January 1, 2017, and the adoption had no impact on its consolidated financial statements. In November 2016, the FASB issued ASU No. 2016-18, Statement of Cash Flows: Restricted Cash, which requires entities to include restricted cash in cash and cash equivalents when reconciling beginning-of-period and end-of-period cash in the statement of cash flows. ASU No. 2016-18 is effective for annual periods beginning after December 15, 2017, including interim periods within those annual periods. The Company early adopted ASU No. 2016-18 retrospectively on January 1, 2017 and the adoption did not have a material impact on its consolidated financial statements, aside from changes in presentation. Recently Issued Accounting Pronouncements — There have been no changes in the expected dates of adoption or estimated effects on the Company's consolidated financial statements of recently issued accounting pronouncements from those disclosed in the Company’s Annual Report on Form 10-K, except as described below. ASU No. 2014-09, Revenue from Contracts with Customers, which creates Accounting Standards Codification ("ASC") 606 and supersedes the existing accounting standards for revenue recognition in ASC 605, provides principles for recognizing revenue for the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods and services. The guidance permits two methods of adoption: (1) full retrospective method, under which the new guidance is applied to each prior reporting period presented; and (2) modified retrospective method, under which the cumulative effect of adoption of the new guidance is applied as an adjustment to the opening balance of retained earnings at the date of initial application. ASU No. 2014-09 is effective for annual reporting periods beginning after December 15, 2017, including interim periods within those annual periods. The Company will adopt the new guidance on January 1, 2018, and expects to use the full retrospective method. The Company will identify performance obligations under ASC 606 that are similar to the deliverables and separate units of account identified under the current guidance in ASC 605. As a result, the timing of revenue recognition for most of its contracts will remain the same. A few of the Company's contracts include contingent amounts of variable consideration that were precluded from recognition under ASC 605 because of the requirement for amounts to be “fixed or determinable." Under ASC 606, the Company will estimate these amounts in advance, resulting in earlier revenue recognition for these contracts. Based on the preliminary calculations of the effects of adopting this standard, the Company anticipates that $4 to $6 million of 2016 revenue would have been recognized in 2015 and prior. The net impact to 2017 revenue is estimated to be immaterial based on contracts in place as of September 30, 2017. ASU No. 2014-09 also creates a new subtopic under ASC 340, Other Assets and Deferred Costs, which discusses the deferral of incremental costs of obtaining a contract with a customer, including the period of amortization of such costs. The Company is currently finalizing its assessment of the planned amortization period for such costs, but has concluded the amortization period will be longer as a result of adopting the new standard. The Company anticipates that the new standard will decrease 2016 and 2017 operating expenses by less than $5 million per year. The final quantitative effects of the adoption of ASC 606 are subject to change from these estimates, pending the completion of the Company’s implementation in the first quarter of 2018. In May 2017, the FASB issued ASU No. 2017-09, Scope of Modification Accounting, which clarifies when a change in the terms or conditions of a share-based payment award should be accounted for as a modification. ASU No. 2017-09 is effective for annual periods beginning after December 15, 2017, including interim periods within those annual periods. The Company will adopt ASU No. 2017-09 on January 1, 2018, and the adoption is not expected to have a material impact on its consolidated financial statements.
Earnings per share	Basic earnings per share is calculated by dividing net income available to common stockholders by the weighted-average number of shares outstanding during the period. The holders of unvested RSAs do not have nonforfeitable rights to dividends or dividend equivalents and therefore, such vested awards do not qualify as participating securities and are excluded from the basic earnings per share calculation. Diluted earnings per share includes the determinants of basic net income per share and, in addition, gives effect to the potential dilution that would occur if securities or other contracts to issue common stock are exercised, vested, or converted into common stock, unless they are anti-dilutive. As the Company intends to settle the principal amount of the Notes (see Note 8, "Debt") in cash upon conversion, their dilutive effect is reflected in diluted earnings per share using the treasury stock method, which considers the number of shares that would be required to settle any premium above principal at the average stock price for the period. During the three and nine months ended September 30, 2016, the average price of the Company's stock was below the conversion price of the Notes; as a result, the Notes were not dilutive for those periods.

Disclosure of accounting policy for trade and other accounts receivable, and finance, loan and lease receivables, including those classified as held for investment and held for sale. This disclosure may include (1) the basis at which such receivables are carried in the entity's statements of financial position (2) how the level of the valuation allowance for receivables is determined (3) when impairments, charge-offs or recoveries are recognized for such receivables (4) the treatment of origination fees and costs, including the amortization method for net deferred fees or costs (5) the treatment of any premiums or discounts or unearned income (6) the entity's income recognition policies for such receivables, including those that are impaired, past due or placed on nonaccrual status and (7) the treatment of foreclosures or repossessions (8) the nature and amount of any guarantees to repurchase receivables.