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Commitments and Contingencies (Details) - USD ($) $ in Millions	Sep. 30, 2019	Mar. 31, 2019
Commitments and Contingencies
Co-investment commitments in partnership	$ 146.3
Affiliate
Commitments and Contingencies
Other commitments	$ 190.0
Equity Method Investee | Non-US
Commitments and Contingencies
Noncontrolling interest, sell of ownership percentage		5.00%
Ownership percentage held interest in affiliate to the Company	14.00%

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- Definition

-- None. No documentation exists for this element. --

+ References

No definition available.

+ Details

Name:	amg_CommitmentAndContingenciesLineItems
Namespace Prefix:	amg_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

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- Definition

The amount that the entity agreed to co-invest in certain investment partnerships where the entity serves as the general partner.

+ References

No definition available.

+ Details

Name:	amg_InvestmentCommitmentAmount
Namespace Prefix:	amg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Noncontrolling Interest, Notification of Possible Sale of Noncontrolling Interest, Percentage

+ References

No definition available.

+ Details

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Namespace Prefix:	amg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Definition

The equity interest of noncontrolling shareholders, partners or other equity holders in consolidated entity.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Minimum amount of other commitment not otherwise specified in the taxonomy. Excludes commitments explicitly modeled in the taxonomy, including but not limited to, long-term and short-term purchase commitments, recorded and unrecorded purchase obligations, supply commitments, registration payment arrangements, leases, debt, product warranties, guarantees, environmental remediation obligations, and pensions.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Details

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Namespace Prefix:
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Balance Type:
Period Type:

X

- Details

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Namespace Prefix:
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Balance Type:
Period Type:

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- Details

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Namespace Prefix:
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Period Type: