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Commitments and Contingencies (Details)	Jun. 30, 2019 USD ($)
Commitments and Contingencies
Co-investment commitments in partnership	$ 139,300,000
Affiliate
Commitments and Contingencies
Other commitments	150,000,000.0
Other commitments, expected payments	$ 0
Equity Method Investee | Non-US
Commitments and Contingencies
Ownership percentage held interest in affiliate to the Company	14.00%

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- Definition

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No definition available.

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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.

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No definition available.

+ Details

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Namespace Prefix:	amg_
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- Definition

Other Commitment Expected Payments

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No definition available.

+ Details

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- Definition

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

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No definition available.

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- 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.

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No definition available.

+ Details

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