Accounting Standards and Policies (Policies)	9 Months Ended
Sep. 30, 2021
Accounting Policies [Abstract]
Basis of Presentation	The Consolidated Financial Statements of Affiliated Managers Group, Inc. (the “Company”) have been prepared in accordance with accounting principles generally accepted in the U.S. (“GAAP”) for interim financial information and with the instructions to Form 10-Q and Rule 10-01 of Regulation S-X. Accordingly, they do not include all of the information and footnotes required by GAAP for full year financial statements. In the opinion of management, all normal and recurring adjustments considered necessary for a fair statement of the Company’s interim financial position and results of operations have been included and all intercompany balances and transactions have been eliminated. Certain reclassifications have been made to the prior period’s financial statements to conform to the current period’s presentation. Operating results for interim periods are not necessarily indicative of the results that may be expected for any other period or for the full year. The Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2020 includes additional information about its operations, financial position, and accounting policies, and should be read in conjunction with this Quarterly Report on Form 10-Q.All amounts in these notes, except per share data in the text and tables herein, are stated in millions unless otherwise indicated
Use of Estimates	The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts and disclosures in the financial statements. Actual results could differ from those estimates.
Recently Adopted Accounting Standards and Recent Accounting Developments	Recently Adopted Accounting Standards Effective January 1, 2021, the Company adopted Accounting Standard Update (“ASU”) 2019-12, Simplifying the Accounting for Income Taxes. The adoption of this standard did not have a significant impact on the Company’s Consolidated Financial Statements. Recent Accounting Developments In August 2020, the Financial Accounting Standards Board (“FASB”) issued ASU 2020-06, Debt with Conversion and Other Options and Derivatives and Hedging - Contracts in Entity’s Own Equity, which simplifies the accounting for convertible instruments and also modifies how particular convertible instruments and certain contracts that may be settled in cash or shares impact the diluted earnings per share calculation. The standard is effective for interim and annual periods beginning after December 15, 2021 for the Company and its consolidated Affiliates, and is effective for interim and annual periods beginning after December 15, 2023 for the Company’s Affiliates accounted for under the equity method. The Company’s adoption of ASU 2020-06 will result in the Company accounting for its convertible debt instrument as a single liability measured at amortized cost and will modify how certain equity instruments that may be settled in cash or shares, at the Company’s option, impact the calculation of Earnings per share (diluted). The Company plans to adopt the standard using a modified retrospective method.
Investments in Affiliates and Affiliate Sponsored Investments Products	Investments in Affiliates and Affiliate Sponsored Investment Products In evaluating whether an investment must be consolidated, the Company evaluates the risk, rewards, and significant terms of each of its Affiliates and other investments to determine if an investment is considered a voting rights entity (“VRE”) or a variable interest entity (“VIE”). An entity is a VRE when the total equity investment at risk is sufficient to enable the entity to finance its activities independently, and when the equity holders have the obligation to absorb losses, the right to receive residual returns, and the right to direct the activities of the entity that most significantly impact its economic performance. An entity is a VIE when it lacks one or more of the characteristics of a VRE, which, for the Company, are Affiliate investments structured as partnerships (or similar entities) where the Company is a limited partner and lacks substantive kick-out or substantive participation rights over the general partner. Assessing whether an entity is a VRE or VIE involves judgment. Upon the occurrence of certain events, management reviews and reconsiders its previous conclusion regarding the status of an entity as a VRE or a VIE. The Company consolidates VREs when it has control over significant operating, financial, and investing decisions of the entity. When the Company lacks such control, but is deemed to have significant influence, the Company accounts for the VRE under the equity method. Other investments with readily determinable fair values in which the Company does not have rights to exercise significant influence are recorded at fair value on the Consolidated Balance Sheets, with changes in fair value included in Investment and other income. The Company consolidates VIEs when it is the primary beneficiary of the entity, which is defined as having the power to direct the activities that most significantly impact the VIE’s economic performance and the obligation to absorb losses of, or the right to receive benefits from, the entity that could potentially be significant to the VIE. Substantially all of the Company’s consolidated Affiliates considered VIEs are controlled because the Company holds a majority of the voting interests or it is the managing member or general partner. Furthermore, an Affiliate’s assets can be used for purposes other than the settlement of the respective Affiliate’s obligations. The Company applies the equity method of accounting to VIEs where the Company is not the primary beneficiary, but has the ability to exercise significant influence over operating and financial matters of the VIE. Investments in Affiliates Substantially all of the Company’s Affiliates are considered VIEs and are either consolidated or accounted for under the equity method. A limited number of the Company’s Affiliates are considered VREs and most of these are accounted for under the equity method. When an Affiliate is consolidated, the portion of the earnings attributable to Affiliate management’s equity ownership is included in Net income (non-controlling interests) in the Consolidated Statements of Income. Undistributed earnings attributable to Affiliate managements’ equity ownership, along with their share of any tangible or intangible net assets, are presented within Non-controlling interests on the Consolidated Balance Sheets. Affiliate equity interests where the holder has certain rights to demand settlement are presented, at their current redemption values, as Redeemable non-controlling interests on the Consolidated Balance Sheets. The Company periodically issues, sells, and purchases the equity of its consolidated Affiliates. Because these transactions take place between entities that are under common control, any gains or losses attributable to these transactions are required to be included in Additional paid-in capital in the Consolidated Balance Sheets, net of any related income tax effects in the period the transaction occurs. When an Affiliate is accounted for under the equity method, the Company’s share of an Affiliate’s earnings or losses, net of amortization and impairments, is included in Equity method income (loss) (net) in the Consolidated Statements of Income and the carrying value of the Affiliate is reported in Equity method investments in Affiliates (net) in the Consolidated Balance Sheets. Deferred taxes recorded on intangible assets upon acquisition of an Affiliate accounted for under the equity method are presented on a gross basis within Equity method investments in Affiliates (net) and Deferred income tax liability (net) in the Consolidated Balance Sheets. The Company’s share of income taxes incurred directly by Affiliates accounted for under the equity method is recorded in Income tax expense in the Consolidated Statements of Income. The Company periodically performs assessments to determine if the fair value of an investment may have declined below its related carrying value for its Affiliates accounted for under the equity method for a period that the Company considers to be other-than-temporary. Where the Company believes that such declines may have occurred, the Company determines the amount of impairment using valuation methods, such as discounted cash flow analyses. Impairments are recorded as an expense in Equity method income (loss) (net) to reduce the carrying value of the Affiliate to its fair value. Affiliate Sponsored Investment Products The Company’s Affiliates sponsor various investment products where the Affiliate also acts as the investment adviser. These investment products are typically owned primarily by third-party investors; however, certain products are funded with general partner and seed capital investments from the Company and its Affiliates. Third-party investors in Affiliate sponsored investment products are generally entitled to substantially all of the economics of these products, except for the asset- and performance-based fees earned by the Company’s Affiliates or any gains or losses attributable to the Company’s or its Affiliates’ investments in these products. As a result, the Company generally does not consolidate these products unless the Company’s or its consolidated Affiliate’s interest in the product is considered substantial. When the Company’s or its consolidated Affiliates’ interests are considered substantial and the products are consolidated, the Company retains the specialized investment company accounting principles of the underlying products, and all of the underlying investments are carried at fair value in Investments in marketable securities in the Consolidated Balance Sheets, with corresponding changes in the investments’ fair values included in Investment and other income. Purchases and sales of securities are presented within purchases and sales by consolidated Affiliate sponsored investment products in the Consolidated Statements of Cash Flows, respectively, and the third-party investors’ interests are recorded in Redeemable non-controlling interests. When the Company or its consolidated Affiliates no longer control these products, due to a reduction in ownership or other reasons, the products are deconsolidated with only the Company’s or its consolidated Affiliate’s investment in the product reported from the date of deconsolidation.The Company’s carrying value, and maximum exposure to loss from unconsolidated Affiliate sponsored investment products, is its or its consolidated Affiliates’ interests in the unconsolidated net assets of the respective products.

Accounting Standards and Policies (Policies)

9 Months Ended

Sep. 30, 2021

The Consolidated Financial Statements of Affiliated Managers Group, Inc. (the “Company”) have been prepared in accordance with accounting principles generally accepted in the U.S. (“GAAP”) for interim financial information and with the instructions to Form 10-Q and Rule 10-01 of Regulation S-X. Accordingly, they do not include all of the information and footnotes required by GAAP for full year financial statements. In the opinion of management, all normal and recurring adjustments considered necessary for a fair statement of the Company’s interim financial position and results of operations have been included and all intercompany balances and transactions have been eliminated. Certain reclassifications have been made to the prior period’s financial statements to conform to the current period’s presentation. Operating results for interim periods are not necessarily indicative of the results that may be expected for any other period or for the full year. The Company’s Annual Report on Form 10-K for the fiscal year ended December 31, 2020 includes additional information about its operations, financial position, and accounting policies, and should be read in conjunction with this Quarterly Report on Form 10-Q.All amounts in these notes, except per share data in the text and tables herein, are stated in millions unless otherwise indicated

Use of Estimates

The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts and disclosures in the financial statements. Actual results could differ from those estimates.

Recently Adopted Accounting Standards and Recent Accounting Developments

Recently Adopted Accounting Standards

Effective January 1, 2021, the Company adopted Accounting Standard Update (“ASU”) 2019-12, Simplifying the Accounting for Income Taxes. The adoption of this standard did not have a significant impact on the Company’s Consolidated Financial Statements.

Recent Accounting Developments

In August 2020, the Financial Accounting Standards Board (“FASB”) issued ASU 2020-06, Debt with Conversion and Other Options and Derivatives and Hedging - Contracts in Entity’s Own Equity, which simplifies the accounting for convertible instruments and also modifies how particular convertible instruments and certain contracts that may be settled in cash or shares impact the diluted earnings per share calculation. The standard is effective for interim and annual periods beginning after December 15, 2021 for the Company and its consolidated Affiliates, and is effective for interim and annual periods beginning after December 15, 2023 for the Company’s Affiliates accounted for under the equity method. The Company’s adoption of ASU 2020-06 will result in the Company accounting for its convertible debt instrument as a single liability measured at amortized cost and will modify how certain equity instruments that may be settled in cash or shares, at the Company’s option, impact the calculation of Earnings per share (diluted). The Company plans to adopt the standard using a modified retrospective method.

Investments in Affiliates and Affiliate Sponsored Investments Products

Investments in Affiliates and Affiliate Sponsored Investment Products

In evaluating whether an investment must be consolidated, the Company evaluates the risk, rewards, and significant terms of each of its Affiliates and other investments to determine if an investment is considered a voting rights entity (“VRE”) or a variable interest entity (“VIE”). An entity is a VRE when the total equity investment at risk is sufficient to enable the entity to finance its activities independently, and when the equity holders have the obligation to absorb losses, the right to receive residual returns, and the right to direct the activities of the entity that most significantly impact its economic performance. An entity is a VIE when it lacks one or more of the characteristics of a VRE, which, for the Company, are Affiliate investments structured as partnerships (or similar entities) where the Company is a limited partner and lacks substantive kick-out or substantive participation rights over the general partner. Assessing whether an entity is a VRE or VIE involves judgment. Upon the occurrence of certain events, management reviews and reconsiders its previous conclusion regarding the status of an entity as a VRE or a VIE.

The Company consolidates VREs when it has control over significant operating, financial, and investing decisions of the entity. When the Company lacks such control, but is deemed to have significant influence, the Company accounts for the VRE under the equity method. Other investments with readily determinable fair values in which the Company does not have rights to exercise significant influence are recorded at fair value on the Consolidated Balance Sheets, with changes in fair value included in Investment and other income.

The Company consolidates VIEs when it is the primary beneficiary of the entity, which is defined as having the power to direct the activities that most significantly impact the VIE’s economic performance and the obligation to absorb losses of, or the right to receive benefits from, the entity that could potentially be significant to the VIE. Substantially all of the Company’s

consolidated Affiliates considered VIEs are controlled because the Company holds a majority of the voting interests or it is the managing member or general partner. Furthermore, an Affiliate’s assets can be used for purposes other than the settlement of the respective Affiliate’s obligations. The Company applies the equity method of accounting to VIEs where the Company is not the primary beneficiary, but has the ability to exercise significant influence over operating and financial matters of the VIE.

Investments in Affiliates

Substantially all of the Company’s Affiliates are considered VIEs and are either consolidated or accounted for under the equity method. A limited number of the Company’s Affiliates are considered VREs and most of these are accounted for under the equity method.

When an Affiliate is consolidated, the portion of the earnings attributable to Affiliate management’s equity ownership is included in Net income (non-controlling interests) in the Consolidated Statements of Income. Undistributed earnings attributable to Affiliate managements’ equity ownership, along with their share of any tangible or intangible net assets, are presented within Non-controlling interests on the Consolidated Balance Sheets. Affiliate equity interests where the holder has certain rights to demand settlement are presented, at their current redemption values, as Redeemable non-controlling interests on the Consolidated Balance Sheets. The Company periodically issues, sells, and purchases the equity of its consolidated Affiliates. Because these transactions take place between entities that are under common control, any gains or losses attributable to these transactions are required to be included in Additional paid-in capital in the Consolidated Balance Sheets, net of any related income tax effects in the period the transaction occurs.

When an Affiliate is accounted for under the equity method, the Company’s share of an Affiliate’s earnings or losses, net of amortization and impairments, is included in Equity method income (loss) (net) in the Consolidated Statements of Income and the carrying value of the Affiliate is reported in Equity method investments in Affiliates (net) in the Consolidated Balance Sheets. Deferred taxes recorded on intangible assets upon acquisition of an Affiliate accounted for under the equity method are presented on a gross basis within Equity method investments in Affiliates (net) and Deferred income tax liability (net) in the Consolidated Balance Sheets. The Company’s share of income taxes incurred directly by Affiliates accounted for under the equity method is recorded in Income tax expense in the Consolidated Statements of Income.

The Company periodically performs assessments to determine if the fair value of an investment may have declined below its related carrying value for its Affiliates accounted for under the equity method for a period that the Company considers to be other-than-temporary. Where the Company believes that such declines may have occurred, the Company determines the amount of impairment using valuation methods, such as discounted cash flow analyses. Impairments are recorded as an expense in Equity method income (loss) (net) to reduce the carrying value of the Affiliate to its fair value.

Affiliate Sponsored Investment Products

The Company’s Affiliates sponsor various investment products where the Affiliate also acts as the investment adviser. These investment products are typically owned primarily by third-party investors; however, certain products are funded with general partner and seed capital investments from the Company and its Affiliates.

Third-party investors in Affiliate sponsored investment products are generally entitled to substantially all of the economics of these products, except for the asset- and performance-based fees earned by the Company’s Affiliates or any gains or losses attributable to the Company’s or its Affiliates’ investments in these products. As a result, the Company generally does not consolidate these products unless the Company’s or its consolidated Affiliate’s interest in the product is considered substantial. When the Company’s or its consolidated Affiliates’ interests are considered substantial and the products are consolidated, the

Company retains the specialized investment company accounting principles of the underlying products, and all of the underlying investments are carried at fair value in Investments in marketable securities in the Consolidated Balance Sheets, with corresponding changes in the investments’ fair values included in Investment and other income. Purchases and sales of securities are presented within purchases and sales by consolidated Affiliate sponsored investment products in the Consolidated Statements of Cash Flows, respectively, and the third-party investors’ interests are recorded in Redeemable non-controlling interests. When the Company or its consolidated Affiliates no longer control these products, due to a reduction in ownership or other reasons, the products are deconsolidated with only the Company’s or its consolidated Affiliate’s investment in the product reported from the date of deconsolidation.The Company’s carrying value, and maximum exposure to loss from unconsolidated Affiliate sponsored investment products, is its or its consolidated Affiliates’ interests in the unconsolidated net assets of the respective products.