Related Party Information - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	0 Months Ended	9 Months Ended
Mar. 26, 2014 acre	Nov. 28, 2014
Related Party Transaction [Line Items]
Cash dividends paid to parent		$ 24.2
Senior PIK Toggle Notes [Member] \| Century Intermediate Holding Company [Member]
Related Party Transaction [Line Items]
Parent company interest payment funded by dividends paid to parent		14.3
Aggregate principal amount of an indirect parent company's Senior PIK Toggle notes		285.0
Cash interest rate percentage		9.75%
PIK interest rate percentage		10.50%
HL and L Property Company [Member] \| Century Intermediate Holding Company [Member]
Related Party Transaction [Line Items]
Proceeds from sale of assets to affiliated company		9.9
WHQ Development [Member]
Related Party Transaction [Line Items]
Purchase price of land	7.4
Area of land purchased	14.48
Purchase price of land per acre	510,000
Equity interest held by unconsolidated related party in development project owner	37.00%
Anticipated year of completion for new world headquarters building		2016
Initial lease term		15 years
Expected annual lease rent		$ 9.5

Stated interest rate percentage when periodic interest is paid in cash.

No definition available.

Name:	am_CashInterestStatedPercentageRate
Namespace Prefix:	am_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Anticipated year of construction completion and when the building will be ready for occupancy.

+ References

No definition available.

+ Details

Name:	am_ConstructionCompletionYear
Namespace Prefix:	am_
Data Type:	xbrli:gYearItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of dividends paid to parent that were used to pay interest on a direct or indirect parent's external borrowings.

+ References

No definition available.

+ Details

Name:	am_InterestPaidOnDebt
Namespace Prefix:	am_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Stated interest rate when periodic interest is paid in the form of additional PIK toggle notes.

+ References

No definition available.

+ Details

Name:	am_PaymentInKindInterestStatedPercentageRate
Namespace Prefix:	am_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The cost per acre for acquisition of land for use.

+ References

No definition available.

+ Details

Name:	am_PaymentsToAcquireLandPerAcre
Namespace Prefix:	am_
Data Type:	us-types:perUnitItemType
Balance Type:	na
Period Type:	instant

- Definition

Unconsolidated related party's equity interest in a service provider, lessor or supplier to the entity.

+ References

No definition available.

+ Details

Name:	am_RelatedPartyOwnershipPercentage
Namespace Prefix:	am_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Area of land held.

+ References

No definition available.

+ Details

Name:	us-gaap_AreaOfLand
Namespace Prefix:	us-gaap_
Data Type:	num:areaItemType
Balance Type:	na
Period Type:	instant

- Definition

This element represents disclosure of the total aggregate cash dividends paid to the entity by consolidated subsidiaries, by unconsolidated subsidiaries, and by 50% or less owned persons accounted for using the equity method.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 3

-Subparagraph (SX 210.12-04(b))

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 235

-Section S99

-Paragraph 2

-Subparagraph (SX 210.9-06)

-URI http://asc.fasb.org/extlink&oid=6882215&loc=d3e537971-122884

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 06

-Article 9

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph b

-Article 12

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

-Paragraph 2

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-Topic 835

-SubTopic 30

-Section 55

-Paragraph 8

-URI http://asc.fasb.org/extlink&oid=34725769&loc=d3e28878-108400

+ Details

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

- Definition

Term of the lessee's leasing arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_LesseeLeasingArrangementsOperatingLeasesTermOfContract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

This element represents the payments that the lessee is obligated to make or can be required to make in connection with a property under the terms of an agreement classified as an operating lease, excluding contingent rentals and a guarantee by the lessee of the lessor's debt and the lessee's obligation to pay (apart from the rental payments) executory costs such as insurance, maintenance, and taxes.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 840

-SubTopic 20

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6453985&loc=d3e41499-112717

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 840

-SubTopic 10

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-Paragraph 40

-Subparagraph (Note 3)

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-Name Accounting Standards Codification

-Topic 840

-SubTopic 10

-Section 25

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=7661868&loc=d3e34039-112682

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The cash outflow for acquisition of land for use; excludes purchases of land held as inventory or as investments.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Investing Activities

-URI http://asc.fasb.org/extlink&oid=6516133

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 13

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3213-108585

+ Details

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

- Definition

Amount of cash inflow from the sale of other assets as part of operating activities.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 25

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3536-108585

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_RelatedPartyTransactionLineItems
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- Details

Name:	us-gaap_RelatedPartyTransactionAxis=am_HlAndLpropertyCompanyMember
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- Details

Name:	us-gaap_RelatedPartyTransactionAxis=am_SeniorPaymentInKindToggleNotesMember
Namespace Prefix:
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- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=am_WorldHeadQuartersDevelopmentMember
Namespace Prefix:
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