Summary of significant accounting policies (Policies)	12 Months Ended
Summary of significant accounting policies (Policies)	Dec. 31, 2016
Accounting Policies [Abstract]
Principles of consolidation	Principles of consolidation The consolidated financial statements include the accounts of all majority-owned subsidiaries.
Use of estimates	Use of estimates The preparation of financial statements in conformity with U.S. generally accepted accounting principles requires management to make estimates and assumptions about future events. These estimates and assumptions affect the amounts of assets and liabilities reported, disclosures about contingent assets and liabilities and the reported amounts of revenue and expenses. Such estimates include the valuation of accounts receivable, inventories, outsourcing assets, marketable software, goodwill and other long-lived assets, legal contingencies, indemnifications, assumptions used in the measurement of progress toward completion for systems integration projects, income taxes and retirement and other post-employment benefits, among others. These estimates and assumptions are based on management’s best estimates and judgment. Management evaluates its estimates and assumptions on an ongoing basis using historical experience and other factors, including the current economic environment, which management believes to be reasonable under the circumstances. Management adjusts such estimates and assumptions when facts and circumstances dictate. As future events and their effects cannot be determined with precision, actual results could differ significantly from these estimates. Changes in those estimates resulting from continuing changes in the economic environment will be reflected in the financial statements in future periods.
Cash equivalents	Cash equivalents All short-term investments purchased with a maturity of three months or less and certificates of deposit which may be withdrawn at any time at the discretion of the company without penalty are classified as cash equivalents.
Inventories	Inventories Inventories are valued at the lower of cost or market. Cost is determined on the first-in, first-out method.
Properties	Properties Properties are carried at cost and are depreciated over the estimated lives of such assets using the straight-line method.
Advertising costs	Advertising costs All advertising costs are expensed as incurred.
Shipping and handling	Shipping and handling Costs related to shipping and handling is included in cost of revenue.
Goodwill	Goodwill Goodwill arising from the acquisition of an entity represents the excess of the cost of acquisition over the fair value of the acquired identifiable assets, liabilities and contingent liabilities of the entity recognized at the date of acquisition. Goodwill is initially recognized as an asset and is subsequently measured at cost less any accumulated impairment losses. Goodwill is held in the currency of the acquired entity and revalued to the closing rate at each balance sheet date. The company tests goodwill for impairment annually in the fourth quarter using data as of September 30th of that year, as well as whenever there are events or changes in circumstances (triggering events) that would more likely than not reduce the fair value of one or more reporting units below its respective carrying amount. The impairment assessment involves a two-step test. In step one, the company compares the fair value of each of its reporting units to their respective carrying value. If the carrying value exceeds fair value, then step two of the impairment test is performed to determine if the implied fair value of the goodwill of the reporting unit exceeds the carrying value of that goodwill. If the carrying value of a reporting unit is zero or negative, the second step of the impairment test shall be performed to measure the amount of impairment loss, if any, when it is more likely than not that a goodwill impairment exists. In this evaluation a company is required to take into consideration certain qualitative factors and whether there are significant differences between the carrying value and the estimated fair value of its assets and liabilities, and the existence of significant unrecognized intangible assets. Goodwill is impaired when the carrying value of the goodwill exceeds its implied value. Impaired goodwill is written down to its implied fair value through a charge to the consolidated statement of income in the period the impairment is identified. We estimate the fair value of each reporting unit using a combination of the income approach and market approach. The income approach incorporates the use of a discounted cash flow method in which the estimated future cash flows and terminal values for each reporting unit are discounted to present value. Cash flow projections are based on management's estimates of economic and market conditions, which drive key assumptions of revenue growth rates, operating margins, capital expenditures and working capital requirements. The discount rate in turn is based on various market factors and specific risk characteristics of each reporting unit. The market approach estimates fair value by applying performance metric multiples to the reporting unit's prior and expected operating performance. The multiples are derived from comparable publicly traded companies with similar operating and investment characteristics as the reporting unit. If the fair value of the reporting unit derived using the income approach is significantly different from the fair value estimate using the market approach, the company reevaluates its assumptions used in the two models. When considering the weighting between the market approach and income approach, we gave more weighting to the income approach. The higher weighting assigned to the income approach took into consideration that the guideline companies used in the market approach generally represent larger diversified companies relative to the reporting units and may have different long term growth prospects, among other factors. In order to assess the reasonableness of the calculated reporting unit fair values, the company also compares the sum of the reporting units' fair values to its market capitalization (per share stock price multiplied by shares outstanding) and calculates an implied control premium (the excess of the sum of the reporting units' fair values over the market capitalization). Estimating the fair value of reporting units requires the use of estimates and significant judgments that are based on a number of factors including actual operating results. It is reasonably possible that the judgments and estimates described above could change in future periods.
Revenue recognition	Revenue recognition Revenue is recognized when persuasive evidence of an arrangement exists, delivery has occurred or services have been rendered, the fee is fixed or determinable, and collectability is probable. Revenue from hardware sales with standard payment terms is recognized upon the passage of title and the transfer of risk of loss. Outside the United States, the company recognizes revenue even if it retains a form of title to products delivered to customers, provided the sole purpose is to enable the company to recover the products in the event of customer payment default and the arrangement does not prohibit the customer’s use of the product in the ordinary course of business. Revenue from software licenses with standard payment terms is recognized at the inception of the initial license term and upon execution of an extension to the license term. The company also enters into multiple-element arrangements, which may include any combination of software, hardware, or services. For example, a client may purchase an enterprise server that includes operating system software. In addition, the arrangement may include post-contract support for the software and a contract for post-warranty maintenance for service of the hardware. These arrangements consist of multiple deliverables, with software and hardware delivered in one reporting period and the software support and hardware maintenance services delivered across multiple reporting periods. In another example, the company may provide desktop managed services to a client on a long term multiple year basis and periodically sell software and hardware products to the client. The services are provided on a continuous basis across multiple reporting periods and the software and hardware products are delivered in one reporting period. To the extent that a deliverable in a multiple-deliverable arrangement is subject to specific guidance, that deliverable is accounted for in accordance with such specific guidance. Examples of such arrangements may include leased hardware which is subject to specific leasing guidance or software which is subject to specific software revenue recognition guidance. In these transactions, the company allocates the total revenue to be earned under the arrangement among the various elements based on a selling price hierarchy. The selling price for a deliverable is based on its vendor specific objective evidence (VSOE) if available, third party evidence (TPE) if VSOE is not available, or the best estimated selling price (ESP) if neither VSOE nor TPE is available. VSOE of selling price is based upon the normal pricing and discounting practices for those services and products when sold separately. TPE of selling price is based on evaluating largely similar and interchangeable competitor products or services in standalone sales to similarly situated customers. ESP is established considering factors such as margin objectives, discounts off of list prices, market conditions, competition and other factors. ESP represents the price at which the company would transact for the deliverable if it were sold by the company regularly on a standalone basis. As mentioned above, some of the company’s multiple-element arrangements may include leased hardware which is subject to specific leasing guidance. Revenue under these arrangements is allocated considering the relative selling prices of the lease and non-lease elements. Lease deliverables include hardware, financing, maintenance and other executory costs, while non-lease deliverables generally consist of non-maintenance services. The determination of the amount of revenue allocated to the lease deliverables begins by allocating revenue to maintenance and other executory costs plus a profit thereon. These elements are generally recognized over the term of the lease. The remaining amounts are allocated to the hardware and financing elements. The amount allocated to hardware is recognized as revenue monthly over the term of the lease for those leases which are classified as operating leases and at the inception of the lease term for those leases which are classified as sales-type leases. The amount of finance income attributable to sales-type leases is recognized on the accrual basis using the effective interest method. For multiple-element arrangements that involve the licensing, selling or leasing of software, for software and software-related elements, the allocation of revenue is based on VSOE. There may be cases in which there is VSOE of fair value of the undelivered elements but no such evidence for the delivered elements. In these cases, the residual method is used to allocate the arrangement consideration. Under the residual method, the amount of consideration allocated to the delivered elements equals the total arrangement consideration less the aggregate VSOE of fair value of the undelivered elements. For multiple-element arrangements that include services or products that (a) do not include the licensing, selling or leasing of software, or (b) contain software that is incidental to the services or products as a whole or (c) contain software components that are sold, licensed or leased with tangible products when the software components and non-software components (i.e., the software and hardware) of the tangible product function together to deliver the tangible product’s essential functionality (e.g., sales of the company’s enterprise-class servers including software and hardware), or some combination of the above, the allocation of revenue is based on the relative selling prices of each of the deliverables in the arrangement based on the selling price hierarchy, discussed above. For multiple-element arrangements that include both software and non-software deliverables, the company allocates arrangement consideration to the software group and to the non-software group based on the relative selling prices of the deliverables in the arrangement based on the selling price hierarchy discussed above. For the software group, arrangement consideration is further allocated using VSOE as described above. The company recognizes revenue on delivered elements only if: (a) any undelivered services or products are not essential to the functionality of the delivered services or products, (b) the company has an enforceable claim to receive the amount due in the event it does not deliver the undelivered services or products, (c) there is evidence of the selling price for each undelivered service or product, and (d) the revenue recognition criteria otherwise have been met for the delivered elements. Otherwise, revenue on delivered elements is recognized as the undelivered elements are delivered. The company evaluates each deliverable in an arrangement to determine whether it represents a separate unit of accounting. A delivered element constitutes a separate unit of accounting when it has standalone value and there is no customer-negotiated refund or return right for the delivered elements. If these criteria are not met, the deliverable is combined with the undelivered elements and the allocation of the arrangement consideration and revenue recognition are determined for the combined unit as a single unit. Revenue from hardware sales and software licenses with extended payment terms is recognized as payments from customers become due (assuming that all other conditions for revenue recognition have been satisfied). Revenue for operating leases is recognized on a monthly basis over the term of the lease and for sales-type leases at the inception of the lease term. Revenue from equipment and software maintenance and post-contract support is recognized on a straight-line basis as earned over the terms of the respective contracts. Cost related to such contracts is recognized as incurred. Revenue and profit under systems integration contracts are recognized either on the percentage-of-completion method of accounting using the cost-to-cost method, or when services have been performed, depending on the nature of the project. For contracts accounted for on the percentage-of-completion basis, revenue and profit recognized in any given accounting period are based on estimates of total projected contract costs. The estimates are continually reevaluated and revised, when necessary, throughout the life of a contract. Any adjustments to revenue and profit resulting from changes in estimates are accounted for in the period of the change in estimate. When estimates indicate that a loss will be incurred on a contract upon completion, a provision for the expected loss is recorded in the period in which the loss becomes evident. Revenue from time and materials service contracts and outsourcing contracts is recognized as the services are provided using either an objective measure of output or on a straight-line basis over the term of the contract.
Income taxes	Income taxes Income taxes are based on income before taxes for financial reporting purposes and reflect a current tax liability for the estimated taxes payable in the current-year tax returns and changes in deferred taxes. Deferred tax assets or liabilities are determined based on differences between financial reporting and tax bases of assets and liabilities and are measured using enacted tax laws and rates. A valuation allowance is provided on deferred tax assets if it is determined that it is more likely than not that the asset will not be realized. The company recognizes penalties and interest accrued related to income tax liabilities in provision for income taxes in its consolidated statements of income.
Marketable software	Marketable software The cost of development of computer software to be sold or leased, incurred subsequent to establishment of technological feasibility, is capitalized and amortized to cost of sales over the estimated revenue-producing lives of the products, but not in excess of three years following product release. The company performs quarterly reviews to ensure that unamortized costs remain recoverable from future revenue.
Internal-use software	Internal-use software The company capitalizes certain internal and external costs incurred to acquire or create internal-use software, principally related to software coding, designing system interfaces, and installation and testing of the software. These costs are amortized in accordance with the fixed asset policy described above.
Outsourcing assets	Outsourcing assets Costs on outsourcing contracts are generally expensed as incurred. However, certain costs incurred upon initiation of an outsourcing contract (principally initial customer setup) are deferred and expensed over the initial contract life. Fixed assets and software used in connection with outsourcing contracts are capitalized and depreciated over the shorter of the initial contract life or in accordance with the fixed asset policy described above. Recoverability of outsourcing assets is subject to various business risks. Quarterly, the company compares the carrying value of the outsourcing assets with the undiscounted future cash flows expected to be generated by the outsourcing assets to determine if there is impairment. If impaired, the outsourcing assets are reduced to an estimated fair value on a discounted cash flow basis. The company prepares its cash flow estimates based on assumptions that it believes to be reasonable but are also inherently uncertain. Actual future cash flows could differ from these estimates.
Translation of foreign currency	Translation of foreign currency The local currency is the functional currency for most of the company’s international subsidiaries, and as such, assets and liabilities are translated into U.S. dollars at year-end exchange rates. Income and expense items are translated at average exchange rates during the year. Translation adjustments resulting from changes in exchange rates are reported in other comprehensive income (loss). Exchange gains and losses on intercompany balances are reported in other income (expense), net. For those international subsidiaries operating in highly inflationary economies, the U.S. dollar is the functional currency, and as such, nonmonetary assets and liabilities are translated at historical exchange rates, and monetary assets and liabilities are translated at current exchange rates. Exchange gains and losses arising from translation are included in other income (expense), net.
Stock-based compensation plans	Stock-based compensation plans Stock-based compensation represents the cost related to stock-based awards granted to employees and directors. The company recognizes compensation expense for the fair value of stock options, which have graded vesting, on a straight-line basis over the requisite service period. The company estimates the fair value of stock options using a Black-Scholes valuation model. The expense is recorded in selling, general and administrative expenses.
Retirement benefits	Retirement benefits Accounting rules covering defined benefit pension plans and other postretirement benefits require that amounts recognized in financial statements be determined on an actuarial basis. A significant element in determining the company’s retirement benefits expense or income is the expected long-term rate of return on plan assets. This expected return is an assumption as to the average rate of earnings expected on the funds invested or to be invested to provide for the benefits included in the projected pension benefit obligation. The company applies this assumed long-term rate of return to a calculated value of plan assets, which recognizes changes in the fair value of plan assets in a systematic manner over four years. This produces the expected return on plan assets that is included in retirement benefits expense or income. The difference between this expected return and the actual return on plan assets is deferred. The net deferral of past asset losses or gains affects the calculated value of plan assets and, ultimately, future retirement benefits expense or income. At December 31 of each year, the company determines the fair value of its retirement benefits plan assets as well as the discount rate to be used to calculate the present value of plan liabilities. The discount rate is an estimate of the interest rate at which the retirement benefits could be effectively settled. In estimating the discount rate, the company looks to rates of return on high-quality, fixed-income investments currently available and expected to be available during the period to maturity of the retirement benefits. The company uses a portfolio of fixed-income securities, which receive at least the second-highest rating given by a recognized ratings agency.
Fair value measurements	Fair value measurements Fair value is defined as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. When determining fair value measurements for assets and liabilities required to be recorded at fair value, the company assumes that the transaction is an orderly transaction that assumes exposure to the market for a period before the measurement date to allow for marketing activities that are usual and customary for transactions involving such assets or liabilities; it is not a forced transaction (for example, a forced liquidation or distress sale). The fair value hierarchy has three levels of inputs that may be used to measure fair value: Level 1 – Quoted prices (unadjusted) in active markets for identical assets or liabilities that the company can access at the measurement date; Level 2 – Inputs other than quoted prices within Level 1 that are observable for the asset or liability, either directly or indirectly; and Level 3 – Unobservable inputs for the asset or liability. The company has applied fair value measurements to its long-term debt (see note 12), derivatives (see note 12) and to its postretirement plan assets (see note 16).
Noncontrolling interest	Noncontrolling interest The company owns a fifty-one percent interest in Intelligent Processing Solutions Ltd. (iPSL), a U.K. business processing outsourcing joint venture. The remaining interests, which are reflected as a noncontrolling interest in the company’s financial statements, are owned by three financial institutions for which iPSL performs services.
Recent accounting pronouncements and accounting changes	Effective January 1, 2016, the company adopted new guidance issued by the Financial Accounting Standards Board ("FASB") on the presentation of debt issuance costs. The new guidance requires that debt issuance costs shall be reported in the balance sheet as a direct deduction from the face amount of that debt. Previously the company reported these costs in “Other long-term assets” in the company’s consolidated balance sheets. At December 31, 2015, the amount reclassified was $1.8 million. The new guidance has been applied on a retrospective basis whereby prior-period financial statements have been adjusted to reflect the application of the new guidance, as required by the FASB. Effective January 1, 2016, the company adopted new guidance issued by the FASB that simplifies the measurement of inventory. The new guidance states that an entity should measure inventory at the lower of cost and net realizable value. Net realizable value is the estimate of estimated selling prices in the ordinary course of business, less reasonably predictable costs of completion, disposal and transportation. When evidence exists that the net realizable value of inventory is lower than its cost, the difference shall be recognized as a loss in the period in which it occurs. That loss may be required, for example, due to damage, physical deterioration, obsolescence, changes in price levels, or other causes. Adoption of this new guidance had no impact on the company’s consolidated results of operations and financial position. Effective January 1, 2016, the company adopted new guidance issued by the FASB that simplifies the balance sheet classification of deferred income taxes. The new guidance requires that deferred tax liabilities and assets be classified as noncurrent in a classified statement of financial position. The new guidance also requires companies to offset all deferred tax assets and liabilities (and valuation allowances) for each tax-paying jurisdiction within each tax-paying component. The net deferred tax must be presented as a single noncurrent amount. Previous guidance required an entity to separate deferred income tax liabilities and assets into current and noncurrent amounts in a classified statement of financial position. The new guidance has been applied on a retrospective basis whereby prior-period financial statements have been adjusted to reflect the application of the new guidance. At December 31, 2015, the reclassification resulted in a reduction of current deferred income tax assets of $24.1 million, a decrease in other current assets of $0.1 million, an increase in noncurrent deferred income tax assets of $12.9 million, a decrease in other long-term assets of $0.1 million, a decrease in current other accrued liabilities of $9.4 million and a decrease in other long-term liabilities of $2.0 million. Effective January 1, 2016, the company adopted new guidance issued by the FASB that removes the requirement to categorize within the fair value hierarchy and make certain disclosures for all investments that are eligible to be measured at fair value using the net asset value per share practical expedient. The new guidance has been applied on a retrospective basis whereby prior-period disclosures have been adjusted to reflect the application of the new guidance. Adoption of this new guidance had no impact on the company’s consolidated results of operations and financial position. Effective for the annual reporting period ended December 31, 2016, the company adopted new guidance issued by the FASB which requires management to evaluate whether there is substantial doubt about an entity's ability to continue as a going concern for each annual and interim reporting period. If substantial doubt exists, additional disclosure is required. Adoption of this new guidance had no impact on the company's consolidated results of operations and financial position. In January 2017, the FASB issued new guidance which simplifies how an entity is required to test goodwill for impairment by eliminating Step 2 from the goodwill impairment test. Step 2 measures a goodwill impairment loss by comparing the implied fair value of a reporting unit's goodwill with the carrying amount of that goodwill. Under the amended guidance, an entity will perform its annual or interim goodwill impairment test by comparing the fair value of a reporting unit with its carrying amount. An impairment charge will be recognized for the amount by which the carrying amount exceeds the reporting unit’s fair value. The guidance is effective for annual reporting periods beginning after December 15, 2019, with early adoption permitted for interim or annual goodwill impairment tests performed on testing dates after January 1, 2017. The company will adopt the new guidance on January 1, 2017. The company does not expect the adoption to have a material impact on its consolidated results of operations and financial position. In October 2016, the FASB issued new guidance which reduces the complexity in the accounting standards by allowing the recognition of current and deferred income taxes for an intra-entity asset transfer, other than inventory, when the transfer occurs. Historically, recognition of the income tax consequence was not recognized until the asset was sold to an outside party. This amendment should be applied on a modified retrospective basis through a cumulative-effect adjustment directly to retained earnings as of the beginning of the period of adoption. This update is effective for annual periods beginning after December 15, 2017, and interim periods within those fiscal years, with earlier adoption permitted. The company will adopt the new guidance on January 1, 2017. The company does not expect the adoption to have a material impact on its consolidated results of operations and financial position. In August 2016, the FASB issued new guidance which clarifies the treatment of several cash flow categories. In addition, the guidance also clarifies that when cash receipts and cash payments have aspects of more than one class of cash flows and cannot be separated, classification will depend on the predominant source or use. This update is effective for annual periods beginning after December 15, 2017, and interim periods within those fiscal years, with early adoption permitted, including adoption in an interim period. The company will adopt the new guidance on January 1, 2017. The company does not expect the adoption to have a material impact on its consolidated statements of cash flows. In June 2016, the FASB issued new guidance that introduces a new model for recognizing credit losses on financial instruments based on an estimate of current expected losses. This includes trade and other receivables, loans and other financial instruments. This update is effective for annual periods beginning after December 15, 2019, with earlier adoption permitted. The company is currently assessing when it will choose to adopt, and is currently evaluating the impact of the adoption on its consolidated financial statements. In March 2016, the FASB issued new guidance that will change certain aspects of accounting for share-based payments to employees. The new guidance will require all income tax effects of awards to be recognized in the income statement when the awards vest or are settled. It also will allow an employer to repurchase more of an employee’s shares than it can today for tax withholding purposes without triggering liability accounting and to make a policy election to account for forfeitures as they occur. The guidance is effective for annual reporting periods beginning after December 15, 2016. The company will adopt the new guidance on January 1, 2017. The company does not expect the adoption to have a material impact on its consolidated results of operations and financial position. In February 2016, the FASB issued a new lease accounting standard entitled “Leases.” The new standard is intended to improve financial reporting about leasing transactions. The new rule will require organizations that lease assets, referred to as lessees, to recognize on the balance sheet the assets and liabilities for the rights and obligations created by those leases. The standard requires disclosures to help investors and other financial statement users better understand the amount, timing and uncertainty of cash flows arising from leases. The standard is effective for annual reporting periods beginning after December 15, 2018, which for the company is January 1, 2019. Earlier adoption is permitted. The company is currently assessing when it will choose to adopt, and is currently evaluating the impact of the adoption on its consolidated results of operations and financial position. In 2014, the FASB issued a new revenue recognition standard entitled “Revenue from Contracts with Customers.” The objective of the standard is to establish the principles that an entity shall apply to report useful information to users of financial statements about the nature, amount, timing, and uncertainty of revenue and cash flows from a contract with a customer. The standard, and its various amendments, is effective for annual reporting periods beginning after December 15, 2017, which for the company is January 1, 2018. Earlier application is permitted only as of annual reporting periods beginning after December 15, 2016, which for the company in January 1, 2017. The standard allows for either “full retrospective” adoption, meaning the standard is applied to all periods presented, or “modified retrospective” adoption, meaning the standard is applied only to the most current period presented in the financial statements. Generally the new standard would require the company to recognize revenue for certain transactions, including extended payment term software licenses and short-term software licenses, sooner than the current rules would allow. The company will adopt the standard on January 1, 2018 using the modified retrospective method. The company is currently evaluating the impact the adoption of this new standard will have on its consolidated results of operations and financial position and currently does not believe that there will be a material impact upon adoption or on a go-forward basis. However, the final impact cannot be determined until the end of 2017 and it will be impacted by transactions entered into during 2017.