XML 103 R89.htm IDEA: XBRL DOCUMENT
v3.8.0.1

Retirement Benefits (Details 13) - Pension Benefits [Member] | 12 Months Ended
--- | ---
Dec. 31, 2017
Minimum [Member] | U.S. equities [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Target asset allocation | 0.18
Minimum [Member] | International equities [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Target asset allocation | .10
Minimum [Member] | Global debt securities and cash [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Target asset allocation | .15
Minimum [Member] | Private equity [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Target asset allocation | 0
Minimum [Member] | Hedge funds [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Target asset allocation | .10
Minimum [Member] | Real estate and other [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Target asset allocation | 0
Maximum [Member] | U.S. equities [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Target asset allocation | .40
Maximum [Member] | International equities [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Target asset allocation | .3
Maximum [Member] | Global debt securities and cash [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Target asset allocation | .4
Maximum [Member] | Private equity [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Target asset allocation | .15
Maximum [Member] | Hedge funds [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Target asset allocation | .2
Maximum [Member] | Real estate and other [Member] |
Defined Benefit Plan Disclosure [Line Items] |
Target asset allocation | 0.1

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_DefinedBenefitPlanDisclosureLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Description of target allocation percentages, or range of percentages, considering the major categories of plan assets as of the latest statement of financial position presented (on a weighted-average basis for employers with more than one plan) that are pertinent to an understanding of the plan investment policies and strategies. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(i) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 5 -Subparagraph (c)(i) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2446-114920 + Details
Name: | us-gaap_DefinedBenefitPlanTargetAllocationPercentage
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RetirementPlanTypeAxis=us-gaap_PensionPlansDefinedBenefitMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=ati_EquitySecuritiesUsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=ati_EquitySecuritiesNonUsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=ati_GlobalcashanddebtMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=us-gaap_PrivateEquityFundsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=us-gaap_HedgeFundsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=us-gaap_RealEstateMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |