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Forward Equity (Details) (Forward Equity Sales, USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013	Dec. 31, 2012
Derivative
Notional amount	$ 70.2	$ 77.0	$ 147.2
Average forward price	198.71	185.56	121.37
Remaining notional amount	252.8
Maximum
Derivative
Notional amount	$ 400.0

X

- Definition

Derivative, Remaining Notional Amount

+ References

No definition available.

+ Details

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Namespace Prefix:	amg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 12

-Section 13

-Sentence Column B

+ Details

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Namespace Prefix:	invest_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

X

- Definition

The average forward price on the group of price risk forward or futures contracts, such as gas futures contracts.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	us-types:perUnitItemType
Balance Type:	na
Period Type:	instant

X

- Details

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Period Type:	duration