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Senior Bank Debt (Details) (USD $) In Millions, unless otherwise specified | 6 Months Ended |
--- | --- | ---
Jun. 30, 2012 | Dec. 31, 2011
Amended credit facility |  |
Senior Bank Debt |  |
Maximum borrowing capacity | $ 1,000.0 |
Revolving credit facility |  |
Senior Bank Debt |  |
Maximum borrowing capacity | 750.0 |
Principal amount maturing on November, 2016 | 720.0 |
Term of debt (in years) | 5 years |
Remaining principal amount maturing on January, 2015 | 30.0 |
Option to increase the borrowing capacity under revolving credit facility | 150.0 |
Line of credit facility amount outstanding | 445.0 | 250.0
Term loan |  |
Senior Bank Debt |  |
Maximum borrowing capacity | 250.0 |
Term of debt (in years) | 5 years |
Option to increase the borrowing capacity under revolving credit facility | $ 250.0 |

X
- Definition Represents the expected term of the debt instrument. + References No definition available. + Details
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Balance Type: | na
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- Definition Represents the option to increase borrowings on the credit facility. + References No definition available. + Details
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Namespace Prefix: | amg_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Amount of long-term debt maturities in the year 2015. + References No definition available. + Details
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Namespace Prefix: | amg_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Amount of long-term debt maturities in the year 2016. + References No definition available. + Details
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Namespace Prefix: | amg_
Data Type: | xbrli:monetaryItemType
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- Details
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- Definition Amount borrowed under the credit facility as of the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 22 -Article 5 + Details
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X
- Definition Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 129 -Paragraph 2, 4 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 22 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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