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Debt - Narrative (Details) |  |  |  |  | 1 Months Ended | 3 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Jun. 02, 2016 USD ($) $ / shares | Sep. 23, 2015 USD ($) | Jun. 02, 2014 USD ($) | Jun. 01, 2014 USD ($) shares | Jul. 31, 2016 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | May 24, 2016 USD ($) | Feb. 15, 2016 | [1] | Dec. 31, 2013 $ / shares
Interest Costs Incurred [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Interest expense |  |  |  |  |  |  |  | $ 125,400,000 | $ 111,400,000 | $ 109,800,000 |  |  |
Interest costs capitalized |  |  |  |  |  |  |  | 4,700,000 | 2,200,000 | 5,300,000 |  |  |
Interest costs paid |  |  |  |  |  |  |  | 127,200,000 | 113,400,000 | 113,200,000 |  |  |
Interest income |  |  |  |  |  |  |  | 1,400,000 | 1,200,000 | 1,100,000 |  |  |
Long-term Debt, Fiscal Year Maturity [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
2017 |  |  |  |  |  |  | $ 105,800,000 | 105,800,000 |  |  |  |  |
2018 |  |  |  |  |  |  | 700,000 | 700,000 |  |  |  |  |
2019 |  |  |  |  |  |  | 350,200,000 | 350,200,000 |  |  |  |  |
2020 |  |  |  |  |  |  | 0 | 0 |  |  |  |  |
2021 |  |  |  |  |  |  | 500,000,000 | 500,000,000 |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Payments of debt issuance costs |  |  |  |  |  |  |  | $ 10,400,000 | 0 | $ 1,200,000 |  |  |
Employer contributions |  |  |  |  |  | $ 115,000,000 |  |  |  |  |  |  |
Allegheny Technologies $500 million 5.875% Senior Notes due 2023 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument interest rate stated percentage |  |  |  |  |  |  |  |  |  |  |  | 7.875% | 5.875%
Increase in interest expense per downgrade notch, percent |  |  |  |  |  |  |  | 0.25% |  |  |  |  |
Maximum number of downgrade notches |  |  |  |  |  |  |  | 4 |  |  |  |  |
Maximum increase in interest rate expense, percent |  |  |  |  |  |  |  | 2.00% |  |  |  |  |
Additional annual interest expense to be incurred |  |  |  |  |  |  |  | $ 1,300,000 | 4,100,000 |  |  |  |
Debt instrument carrying amount | [1] |  |  |  |  |  | $ 500,000,000 | $ 500,000,000 | 500,000,000 |  |  |  |
Allegheny Technologies $500 million 5.875% Senior Notes due 2023 [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument interest rate stated percentage |  |  |  |  |  |  | 7.875% | 7.875% |  |  |  |  |
Domestic Bank Group $400 million credit agreement [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Average outstanding amount |  |  |  |  |  |  |  | $ 82,000,000 | $ 61,000,000 |  |  |  |
Interest rate during period |  |  |  |  |  |  |  | 1.757% | 2.00% |  |  |  |
Domestic Bank Group $400 million asset-based credit facility [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Payments of debt issuance costs |  |  |  |  |  |  |  | $ 900,000 |  |  |  |  |
Debt instrument carrying amount |  |  |  |  |  |  | $ 0 | 0 | $ 0 |  |  |  |
Line of credit facility amount outstanding |  |  |  |  |  |  | 25,900,000 | 25,900,000 |  |  |  |  |
2017 Term Loan [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Payments of debt issuance costs |  |  |  |  |  |  |  | 1,000,000 |  |  |  |  |
Debt instrument carrying amount |  |  |  |  |  |  | $ 100,000,000 | $ 100,000,000 | 0 |  |  |  |
2017 Term Loan [Member] | London Interbank Offered Rate (LIBOR) [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Variable rate spread |  |  |  |  |  |  | 3.50% |  |  |  |  |  |
Allegheny Technologies, Convertible Senior Notes, 4.75%, Due 2022 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument interest rate stated percentage |  |  |  |  |  |  | 4.75% | 4.75% |  |  |  |  |
Debt instrument carrying amount |  |  |  |  |  |  | $ 287,500,000 | $ 287,500,000 | $ 0 |  |  |  |
Allegheny Technologies, Convertible Senior Notes, 4.75%, Due 2022 [Member] | Convertible Debt [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument interest rate stated percentage |  |  |  |  |  |  | 4.75% | 4.75% |  |  |  |  |
Payments of debt issuance costs |  | $ 9,400,000 |  |  |  |  |  |  |  |  |  |  |
Amortization period of debt issuance costs |  |  |  |  |  |  | 6 years |  |  |  |  |  |
Long-term debt |  | $ 37,500,000 |  |  |  |  |  |  |  |  | $ 250,000,000 |  |
Convertible debt conversion rate in shares of stock |  | 0.0692042 |  |  |  |  |  |  |  |  |  |  |
Number of equity instruments issuable |  | 19,900,000 |  |  |  |  |  |  |  |  |  |  |
Conversion price of convertible notes | $ / shares |  | $ 14.45 |  |  |  |  |  |  |  |  |  |  |
Allegheny Technologies $402.5 million 4.25% Convertible Senior Notes due 2014 [Member] | Convertible Debt [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument interest rate stated percentage |  |  |  |  |  |  |  |  |  | 4.25% |  |  |
Debt instrument carrying amount |  |  |  |  |  |  |  |  |  | $ 402,500,000 |  |  |
Convertible debt conversion rate in shares of stock |  |  |  |  | 0.0239263 |  |  |  |  |  |  |  |
Conversion price of convertible notes | $ / shares |  |  |  |  |  |  |  |  |  |  |  |  | $ 41.795
Repayments of convertible debt |  |  |  | $ 397,500,000 |  |  |  |  |  |  |  |  |
Amount of debt converted |  |  |  |  | $ 5,000,000 |  |  |  |  |  |  |  |
Shares issued for converted debt | shares |  |  |  |  | 120,476 |  |  |  |  |  |  |  |
Allegheny Technologies $350 million 9.375% Senior Notes due 2019 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument interest rate stated percentage |  |  |  |  |  |  | 9.375% | 9.375% | 9.375% |  |  |  |
Debt instrument carrying amount |  |  |  |  |  |  | $ 350,000,000 | $ 350,000,000 | $ 350,000,000 |  |  |  |
Ladish Series B 6.14% Notes due 2016 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument interest rate stated percentage |  |  |  |  |  |  |  |  |  | 6.14% |  |  |
Repayments of debt |  |  |  |  |  |  |  |  | 5,700,000 |  |  |  |
Ladish Series C 6.41% Notes due 2015 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument interest rate stated percentage |  |  |  |  |  |  |  |  |  | 6.41% |  |  |
Repayments of debt |  |  |  |  |  |  |  |  | $ 10,000,000 |  |  |  |
Seperate Letter of Credit Facility [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Line of credit facility amount outstanding |  |  |  |  |  |  | 17,000,000 | 17,000,000 |  |  |  |  |
STAL Revolving Credit Facility [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Line of credit maximum borrowing capacity |  |  |  |  |  |  | $ 20,000,000 | $ 20,000,000 |  |  |  |  |
Joint venture ownership percentage |  |  |  |  |  |  | 60.00% | 60.00% |  |  |  |  |
Revolving Credit Facility [Member] | Domestic Bank Group $400 million credit agreement [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Line of credit maximum borrowing capacity |  |  |  |  |  |  |  |  |  | $ 400,000,000 |  |  |
Revolving Credit Facility [Member] | Domestic Bank Group $400 million asset-based credit facility [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Line of credit maximum borrowing capacity |  |  | $ 400,000,000 |  |  |  |  |  |  |  |  |  |
Payments of debt issuance costs |  |  | $ 1,500,000 |  |  |  |  |  |  |  |  |  |
Amortization period of debt issuance costs |  |  | 5 years |  |  |  |  |  |  |  |  |  |
Fixed charge coverage ratio |  |  |  |  |  |  | 1 | 1 |  |  |  |  |
Remaining borrowing capacity, percent of minimum borrowing capacity |  |  |  |  |  |  | 12.50% | 12.50% |  |  |  |  |
Line of Credit Facility, Remaining Borrowing Capacity |  |  |  |  |  |  | $ 62,500,000 | $ 62,500,000 |  |  |  |  |
Minimum remaining borrowing capacity |  |  |  |  |  |  | 40,000,000 | 40,000,000 |  |  |  |  |
Minimum required liquidity |  |  |  |  |  |  | $ 500,000,000 | $ 500,000,000 |  |  |  |  |
Minimum required liquidity, number of days prior |  |  |  |  |  |  |  | 91 days |  |  |  |  |
Revolving Credit Facility [Member] | Domestic Bank Group $400 million asset-based credit facility [Member] | Minimum [Member] | London Interbank Offered Rate (LIBOR) [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Variable rate spread |  |  |  |  |  |  |  | 2.00% |  |  |  |  |
Revolving Credit Facility [Member] | Domestic Bank Group $400 million asset-based credit facility [Member] | Minimum [Member] | Base Rate [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Variable rate spread |  |  |  |  |  |  |  | 1.00% |  |  |  |  |
Revolving Credit Facility [Member] | Domestic Bank Group $400 million asset-based credit facility [Member] | Maximum [Member] | London Interbank Offered Rate (LIBOR) [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Variable rate spread |  |  |  |  |  |  |  | 2.50% |  |  |  |  |
Revolving Credit Facility [Member] | Domestic Bank Group $400 million asset-based credit facility [Member] | Maximum [Member] | Base Rate [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Variable rate spread |  |  |  |  |  |  |  | 1.50% |  |  |  |  |
Letter of Credit [Member] | Domestic Bank Group $400 million asset-based credit facility [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Line of credit maximum borrowing capacity |  |  | $ 200,000,000 |  |  |  |  |  |  |  |  |  |
[1] | Bearing interest at 7.875% effective February 15, 2016.

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- Definition The value of the financial instrument(s) that the original debt is being converted into in a noncash (or part noncash) transaction. "Part noncash" refers to that portion of the transaction not resulting in cash receipts or cash payments in the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4304-108586 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4332-108586 + Details
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- Definition The number of shares issued in exchange for the original debt being converted in a noncash (or part noncash) transaction. "Part noncash" refers to that portion of the transaction not resulting in cash receipts or payments in the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4332-108586 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4304-108586 + Details
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- Definition Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt. + References Reference 1: 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(22)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(16)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(16)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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- Definition Ratio applied to the conversion of debt instrument into equity with equity shares divided by debt principal amount. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21506-112644 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Conversion Rate -URI http://asc.fasb.org/extlink&oid=6509012 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 210.5-02.22(a)(5)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21475-112644 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21521-112644 + Details
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- Definition Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
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- Definition Amount of contributions made by the employer to defined benefit plans. + 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)(5)(ii) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 60 -Section 35 -Paragraph 123 -Subparagraph B -URI http://asc.fasb.org/extlink&oid=64937487&loc=d3e36068-114959 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Section 35 -Paragraph 63 -Subparagraph B -URI http://asc.fasb.org/extlink&oid=64937038&loc=d3e12551-114930 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(5)(iv)(01) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 + Details
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- Definition Amount of interest capitalized during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Allowance for Funds Used during Construction -URI http://asc.fasb.org/extlink&oid=6504829 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6450988&loc=d3e26243-108391 + Details
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- Definition Amount of the cost of borrowed funds accounted for as interest expense for debt. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 225 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.8) -URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688 + Details
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- Definition The effective interest rate during the reporting period. + References Reference 1: 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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- 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 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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