| Significant Accounting Policies (Policies) | 12 Months Ended |
| Dec. 31, 2024 |
| Accounting Policies [Abstract] | |
| Basis of Accounting, Policy [Policy Text Block] | Basis of Presentation Our consolidated financial statements have been prepared according to U.S. GAAP and include all majority-owned subsidiaries and consolidated seeded investment products. Intercompany accounts and transactions have been eliminated in consolidation. Events subsequent to the balance sheet date have been evaluated for inclusion in the accompanying consolidated financial statements through the issuance date. Certain prior year amounts have been reclassified to conform to current year presentation with no |
| New Accounting Pronouncements, Policy [Policy Text Block] | Recent Accounting Pronouncements Recently adopted accounting pronouncements In November 2023, 2023 07, 280 2023 07 January 1, 2024, January 1, 2025. 2024 Recently issued accounting pronouncements not In December 2023, 2023 09, 740 2023 09 January 1, 2025. not In November 2024, 2024 03, 220 40 2024 03 January 1, 2027 January 1, 2028. |
| Use of Estimates, Policy [Policy Text Block] | Accounting Estimates The preparation of consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements, and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates, and the differences could be material. Our significant estimates relate to investments, goodwill and intangible assets, retirement benefit assets and obligations, and income taxes. |
| Segment Reporting, Policy [Policy Text Block] | Segment Information We are a global asset manager and manage a range of investment products, operating across various product lines, distribution channels and geographic regions. However, resources are allocated and the business is managed by the chief operating decision-maker, the CEO, on an aggregated basis. Strategic and financial management decisions are determined centrally by the CEO and, on this basis, we operate as a single-segment investment management business. |
| Consolidation, Policy [Policy Text Block] | Consolidation of Investment Products We perform periodic consolidation analyses of our seeded investment products to determine if the product is a VIE or a VRE. Factors considered in this assessment include the product’s legal organization, the product’s capital structure and equity ownership, and any de facto agent implications of our involvement with the product. Investment products that are determined to be VIEs are consolidated if we are the primary beneficiary of the products. VREs are consolidated if we hold the majority voting interest. Upon the occurrence of certain events (such as contributions and redemptions, either by JHG or third no Variable Interest Entities Certain investment products for which a controlling financial interest is achieved through arrangements that do not not not We consolidate a VIE if we are the VIE’s primary beneficiary. The primary beneficiary of a VIE is defined as the variable interest holder that has a controlling financial interest in the VIE. A controlling financial interest is defined as (i) the power to direct the activities of the VIE that most significantly impact its economic performance and (ii) the obligation to absorb losses of the product or the right to receive benefits from the product that potentially could be significant to the VIE. We are the manager of various types of seeded investment products, which may VIEs are generally subject to consolidation by us when we hold an economic interest of greater than 9% and we deconsolidate such VIEs once equity ownership equals or falls below 9%. VIEs are subject to specific disclosure requirements. Voting Rights Entities We consolidate seeded investment products accounted for as VREs when we are considered to control such products, which generally exist if we have a greater than 50% |
| Property, Plant and Equipment, Policy [Policy Text Block] | Property, Equipment and Software Property, equipment and software are recorded at cost. Depreciation is recorded using the straight-line method over the estimated useful life of the related assets (or the lease term, if shorter). Computer software is recorded at cost and depreciated over its estimated useful life. Internal and external costs incurred in connection with researching or obtaining computer software for internal use are expensed as incurred during the preliminary project stage, as are post-implementation training and maintenance costs. Internal and external costs incurred for internal use software during the application development stage are capitalized until such time that the software is substantially complete and ready for its intended use. Application development stage costs are depreciated on a straight-line basis over the estimated useful life of the software. An impairment loss is recognized if the carrying value of the asset exceeds the fair value of the asset. The amount of the impairment loss is equal to the excess of the carrying amount over the fair value. The evaluation is based on an estimate of the future cash flows expected to result from the use of the asset and its eventual disposal. If expected future undiscounted cash flows are less than the carrying amount of the asset, an impairment loss is recognized in an amount equal to the excess of the carrying amount of the asset over the fair value of the asset. There were no December 31, 2024, 2023 2022 |
| Goodwill and Intangible Assets, Intangible Assets, Policy [Policy Text Block] | Cloud Computing Arrangements Costs paid to vendors for third |
| Debt, Policy [Policy Text Block] | Debt Long-term debt consists of senior notes and is stated at amortized cost using the effective interest rate method. Amortized cost is calculated by taking into account any issuance costs and any discount or premium on settlement. Debt will cease to be recognized when the obligation under the liability has been discharged or cancelled or has expired. |
| Investment, Policy [Policy Text Block] | Investments Seeded Investment Products We periodically add new investment strategies to our investment product offerings by providing the initial cash investment (“seed capital”). The primary purpose of seed capital is to generate an investment performance track record in a product to attract third not third not may third Refer to the Consolidation of Investment Products section in this note for information regarding the consolidation of certain seeded investment products. We may third Investments in Advised Mutual Funds and Investments Related to the Economic Hedging of Deferred Compensation We grant mutual fund share awards to employees that are indexed to certain funds managed by us. Upon vesting, participants receive the value of the mutual fund share awards adjusted for gains or losses attributable to the mutual funds to which the award was indexed, subject to tax withholding, or participants receive shares in the mutual fund. When investments in our fund products are purchased and held against deferred compensation liabilities, any movement in the fair value of the assets and corresponding movements in the deferred compensation liability are recognized within the Consolidated Statements of Comprehensive Income. We maintain deferred compensation plans for certain highly compensated employees and members of the Board of Directors. Eligible participants may no Equity Method Investments Our investment in equity method investees, where we do not 20% 50% Investments are initially recognized at cost when purchased for cash or at the fair value of shares received where acquired as part of a wider transaction. The investments are subsequently carried at cost adjusted for our share of net income or loss and other changes in comprehensive income of the equity method investee, less any dividends or distributions received by us. The Consolidated Statements of Comprehensive Income includes our share of net income or loss for the year, or period of ownership, if shorter, within investment gains (losses), net. |
| Accounts Receivable [Policy Text Block] | Trade Receivables Trade receivables are initially recognized at fair value, which is normally equivalent to the invoice amount. When the time value of money is material, the fair value is discounted. Provision for specific doubtful accounts is made when there is evidence that we may not |
| Receivables and Payables [Policy Text Block] | OEIC and Unit Trust Receivables and Payables OEIC and unit trust receivables and payables are in relation to the purchase of units/shares (by investors) and the liquidation of units/shares (owned by trustees). The amounts are dependent on the level of trading and fund switches in the four |
| Cash and Cash Equivalents, Policy [Policy Text Block] | Cash and Cash Equivalents Cash and cash equivalents primarily consist of cash held at banks, on-demand deposits, investments in money market instruments, highly liquid short-term debt securities and commercial paper with a maturity date of three not not |
| Derivatives, Policy [Policy Text Block] | Derivative Instruments We may, not Derivative instruments are measured at fair value and classified as either other current assets or accounts payable and accrued liabilities in our Consolidated Balance Sheets. Changes in the fair value of derivative instruments are recorded within investment gains (losses), net in our Consolidated Statements of Comprehensive Income. Our consolidated seed investments may |
| Lessee, Leases [Policy Text Block] | Leases We determine if an arrangement is a lease at inception. Operating lease right-of-use (“ROU”) assets are included in other non-current assets within our Consolidated Balance Sheets. The current and non-current portions of operating lease liabilities are included within accounts payable and accrued liabilities and within other non-current liabilities, respectively. Finance lease ROU assets are included within property, equipment and software, net. The current and non-current portions of finance lease liabilities are included within accounts payable and accrued liabilities and within other non-current liabilities, respectively. ROU assets represent our right to use an underlying asset for the lease term, and lease liabilities represent our obligation to make lease payments arising from the lease. Operating lease ROU assets and liabilities are recognized at commencement date based on the present value of lease payments over the lease term. As most of our leases do not may |
| Noncontrolling Interests [Policy Text Block] | Nonredeemable Noncontrolling Interests and Redeemable Noncontrolling Interests Nonredeemable noncontrolling interests that are not |
| Fair Value Measurement, Policy [Policy Text Block] | Fair Value Measurements Fair value is the price that would be received to sell an asset or transfer a liability in an orderly transaction between market participants at the measurement date. The fair value of financial instruments traded in active markets (such as publicly traded securities and derivatives) is based on quoted market prices at the reporting date. The quoted market price used for financial instruments is the last traded market price for both financial assets and financial liabilities where the last traded price falls within the bid ask spread. In circumstances where the last traded price is not not Measurements of fair value are classified within a hierarchy that prioritizes the inputs to valuation techniques used to measure fair value based on whether the inputs to those valuation techniques are observable or unobservable. The valuation hierarchy contains three ● Level 1 ● Level 2 not ● Level 3 The valuation of an asset or liability may one Level 1 Our Level 1 Level 2 Our Level 2 2 Level 3 Our assets and liabilities measured at Level 3 Details of inputs used to calculate the fair value of contingent deferred consideration can be found in Note 10 8, Nonrecurring Fair Value Measurements Nonrecurring Level 3 3. Investments Measured at Net Asset Values As a practical expedient, the Company uses the NAV as the fair value for certain investments. Investments in fund products for which fair value is estimated using the NAV as a practical expedient (when the NAV is available to the Company as an investor but is not not |
| Income Tax, Policy [Policy Text Block] | Income Taxes We provide for current tax expense according to the tax laws in each jurisdiction in which we operate, using tax rates and laws that have been enacted by the balance sheet date. Deferred income tax assets and liabilities are recorded for temporary differences between the financial statement and income tax basis of assets and liabilities as measured by the enacted income tax rates that may We periodically assess the recoverability of our deferred tax assets and the need for valuation allowances on these assets. We make these assessments based on the weight of available evidence regarding possible sources of future taxable income and estimates relating to the future performance of the business that results in taxable income. In evaluating uncertain tax positions, we consider the probability that the tax benefit can be sustained on examination by a taxing authority on the basis of its technical merits (“the recognition threshold”). For tax positions meeting this threshold, the amount recognized within the financial statements is the benefit expected to be realized upon settlement with the taxing authority on the basis of a cumulative-probability assessment of the possible outcomes. For tax positions not no |
| Revenue from Contract with Customer [Policy Text Block] | Revenue Recognition Revenue is determined based on the transaction price negotiated with the customer, net of rebates. Management fees, performance fees, shareowner servicing fees and other revenue are derived from providing professional services to manage investment products. Management fees are earned over time as services are provided and are generally based on a percentage of the market value of AUM. These fees are calculated as a percentage of either the daily, month-end or quarter-end average asset balance in accordance with contractual agreements. Performance fees are specified in certain fund and client contracts and are based on investment performance either on an absolute basis or compared to an established index over a specified period of time. Performance fees are generated on certain management contracts when performance hurdles or other specified criteria are achieved. Performance fees for all fund ranges and other investment products are recognized when it is probable that a significant reversal of revenue recognized will not no no Management fees are primarily earned monthly or quarterly, while performance fees are usually earned monthly, quarterly or annually, although the frequency of receipt varies between agreements. Management and performance fee revenue earned but not Shareowner servicing fees are earned for services rendered related to transfer agent and administrative activities performed for investment products. These services are transferred over time and are generally based on a percentage of the market value of AUM. Other revenue includes distribution and servicing fees earned from U.S. mutual funds associated with mutual fund transfer agent, accounting, shareholder servicing and participant recordkeeping activities. These services are transferred over time and are generally based on a percentage of the market value of AUM. U.S. Mutual Fund Performance Fees The investment management fee paid by each U.S. mutual fund subject to a performance fee is the base management fee plus or minus a performance fee adjustment as determined by the relative investment performance of the fund compared to a specified benchmark index. Under the performance-based fee structure, the investment advisory fee paid by each fund consists of two The addition of performance fees to all funds without such fees is subject to the approval of both a majority of the shareholders of such funds and the funds’ independent board of trustees. Principal Versus Agent We use third third third third |
| Selling, General and Administrative Expenses, Policy [Policy Text Block] | Operating Expenses Operating expenses are accrued and recognized as incurred. |
| Share-Based Payment Arrangement [Policy Text Block] | Stock-Based Compensation We grant stock-based awards to certain employees, all of which are classified as equity settled stock-based payments. Equity settled stock-based payments are measured at the fair value of the shares at the grant date. The awards are expensed, with a corresponding increase in equity, on a graded basis over the vesting period. Forfeitures are recognized as they occur. The grant date fair value for stock options is determined using the Black-Scholes option pricing model, and the grant date fair value of restricted stock is determined from the market price on the date of grant. The Black-Scholes model requires management to determine certain variables; the assumptions used in the Black-Scholes option pricing model include dividend yield, expected volatility, risk-free interest rate and expected life. The dividend yield and expected volatility are determined using historical Company data. The risk-free interest rate for options granted is based on the three three We generally use the Monte Carlo model to determine the fair value of performance-based awards with market conditions. The assumptions used in the Monte Carlo model include dividend yield, share price volatility and discount rate. |
| Commissions, Policy [Policy Text Block] | Commissions Commissions on management fees are accounted for on an accrual basis and are recognized in the accounting period in which the associated management fee is earned. |
| Earnings Per Share, Policy [Policy Text Block] | Earnings Per Share Basic earnings per share attributable to our shareholders is calculated by dividing net income (adjusted for the allocation of earnings to participating restricted stock awards) by the weighted-average number of shares outstanding. We calculate earnings per share using the two two Diluted earnings per share is calculated in a similar way to basic earnings per share but is adjusted for the effect of potential common shares unless they are anti-dilutive. |
| Goodwill and Intangible Assets, Policy [Policy Text Block] | Goodwill and Intangible Assets Goodwill represents the excess of cost over the fair value of the identifiable net assets of acquired companies and is capitalized within the Consolidated Balance Sheets. Intangible assets consist primarily of investment management contracts and trademarks acquired as part of business combinations. Investment management contracts have been identified as separately identifiable intangible assets arising on the acquisition of subsidiaries or businesses. Such contracts are recognized at the present value of the expected future cash flows of the investment management contracts at the date of acquisition. Investment management contracts may Indefinite-lived intangible assets comprise investment management agreements where the agreements are with investment companies themselves and not not Goodwill and indefinite-lived intangible assets are reviewed for impairment annually or more frequently if changes in circumstances indicate that the carrying value may may not may first Goodwill and intangible assets require significant management estimates and judgment, including the valuation and expected life determination upon inception and the ongoing evaluation for impairment. |
| Foreign Currency Transactions and Translations Policy [Policy Text Block] | Foreign Currency Transactions in foreign currencies are recorded at the appropriate exchange rate prevailing at the date of the transaction. Foreign currency monetary balances at the reporting date are converted at the prevailing exchange rate. Foreign currency non-monetary balances carried at fair value or cost are translated at the rates prevailing at the date when the fair value or cost is determined. Gains and losses arising on remeasurement are recognized as a component of net income. On consolidation, the assets and liabilities of our operations for which the functional currency is not |
| Pension and Other Postretirement Plans, Policy [Policy Text Block] | Post-Employment Retirement Benefits We provide employees with retirement benefits through a defined contribution plan. Additionally, we provide a defined benefit contribution to certain employees; however, the defined benefit contribution plan is closed to new members. The assets of these plans are held separately from our general assets in trustee-administered funds. Contributions to the defined contribution plan are expensed to employee compensation and benefits on the Consolidated Statements of Comprehensive Income when they become payable. Defined benefit obligations and the cost of providing benefits are determined annually by independent qualified actuaries using the projected unit credit method. Our annual measurement date of the defined benefit plan is December 31. Actuarial gains and losses arise as a result of the difference between actual experience and actuarial assumptions. We have adopted the 10% 10% no no Net periodic benefit cost is recorded as a component of net income within the Consolidated Statements of Comprehensive Income and includes service cost, interest cost, expected return on plan assets and any actuarial gains and losses previously recognized as a component of other comprehensive income that have been amortized in the period. Net periodic benefit costs, with the exception of service costs, are recognized within other non-operating income (expense), net on the Consolidated Statements of Comprehensive Income; service costs are recognized within employee compensation and benefits. |
| Stockholders' Equity, Policy [Policy Text Block] | Common Stock JHG’s ordinary shares, par value $1.50 per share, are classified as equity instruments. Equity shares issued by us are recorded at the fair value of the proceeds received or the market price on the day of issue. Direct issue costs, net of tax, are deducted from additional paid-in-capital within equity. Treasury shares held are equity shares of JHG acquired by or issued to employee benefit trusts. Treasury shares held are recorded at cost and are deducted from equity. No |
