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Stockholders' Equity (Details) $ / shares in Units, $ in Millions |  | 1 Months Ended | 12 Months Ended
--- | --- | --- | ---
Nov. 07, 2017 $ / shares shares | Feb. 28, 2017 shares | Dec. 31, 2018 USD ($) $ / shares shares | Dec. 31, 2017 USD ($) $ / shares shares | Dec. 31, 2016 USD ($) $ / shares shares | Dec. 31, 2015 USD ($) $ / shares shares | Dec. 31, 2014 $ / shares shares
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract] |  |  |  |  |  |  |
Preferred stock, issued |  |  |  | 0 | 0 |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Shares issued | 17,000,000 |  |  |  |  |  |
Price per share of shares issued (in dollars per share) | $ / shares | $ 24.00 |  |  |  |  |  |
Issuance of common stock | $ |  |  |  | $ 397.8 | $ 0.0 | $ 0.0 |
Granted (shares) |  |  |  | 378,000 | 682,000 | 669,000 |
Common stock shares available for future awards |  |  |  | 5,600,000 |  |  |
Shares vested |  |  |  | 640,000 | 154,000 | 23,000 |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] |  |  |  |  |  |  |
Nonvested beginning of the year (shares) |  |  | 1,320,000 | 1,852,000 | 1,652,000 | 1,376,000 |
Granted (shares) |  |  |  | 378,000 | 682,000 | 669,000 |
Vested (shares) |  |  |  | (640,000) | (154,000) | (23,000) |
Forfeited (shares) |  |  |  | (270,000) | (328,000) | (370,000) |
Nonvested end of the year (shares) |  |  |  | 1,320,000 | 1,852,000 | 1,652,000 | 1,376,000
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward] |  |  |  |  |  |  |
Beginning balance (in dollars per share) | $ / shares |  |  | $ 32,500,000 | $ 51,500,000 | $ 57,000,000 | $ 47,800,000 |
Granted (in dollars per share) | $ / shares |  |  |  | 7,100,000 | 8,400,000 | 20,800,000 |
Vested (in dollars per share) | $ / shares |  |  |  | (18,400,000) | (4,300,000) | (800,000) |
Forfeited (in dollars per share) | $ / shares |  |  |  | (7,700,000) | (9,600,000) | (10,800,000) |
Ending balance (in dollars per share) | $ / shares |  |  |  | $ 32,500,000 | $ 51,500,000 | $ 57,000,000 | $ 47,800,000
Retained Earnings Note Disclosure [Abstract] |  |  |  |  |  |  |
Undistributed earnings of investees accounted for under equity method | $ |  |  |  | $ 10.0 |  |  |
Restricted Stock [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Income target attainment measurement period |  |  |  | 3 years |  |  |
Share based compensation expense | $ |  |  |  |  |  | $ 5.5 |
Restricted stock and RSUs [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Share based compensation expense | $ |  |  |  | $ 9.8 | $ 11.2 |  |
Unrecognized compensation expense | $ |  |  |  | $ 7.3 |  |  |
Performance Awards [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Granted (shares) |  |  |  | 590,000 | 937,000 |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] |  |  |  |  |  |  |
Nonvested beginning of the year (shares) |  |  | 1,441,000 | 919,000 | 0 |  |
Granted (shares) |  |  |  | 590,000 | 937,000 |  |
Forfeited (shares) |  |  |  | (68,000) | (18,000) |  |
Nonvested end of the year (shares) |  |  |  | 1,441,000 | 919,000 | 0 |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward] |  |  |  |  |  |  |
Beginning balance (in dollars per share) | $ / shares |  |  | $ 22.7 | $ 11.3 | $ 0.0 |  |
Granted (in dollars per share) | $ / shares |  |  |  | 12.4 | 11.5 |  |
Forfeited (in dollars per share) | $ / shares |  |  |  | (1.0) | (0.2) |  |
Ending balance (in dollars per share) | $ / shares |  |  |  | $ 22.7 | $ 11.3 | $ 0.0 |
2016 LTIP [Member] | Restricted Share Units [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Granted (shares) |  |  |  |  | 587,661 |  |
Shares vested |  | 190,421 |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] |  |  |  |  |  |  |
Granted (shares) |  |  |  |  | 587,661 |  |
Vested (shares) |  | (190,421) |  |  |  |  |
2017 RSU [Member] | Restricted Share Units [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Granted (shares) |  |  |  | 320,679 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] |  |  |  |  |  |  |
Granted (shares) |  |  |  | 320,679 |  |  |
Long-Term Shareholder Value (LTSV) 2015 [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Percent of performance metric expected to be obtained |  |  |  | 60.00% |  |  |
Shares vested |  |  |  | 122,882 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] |  |  |  |  |  |  |
Vested (shares) |  |  |  | (122,882) |  |  |
Long-Term Shareholder Value (LTSV) 2015 [Member] | Restricted Stock [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Award vesting period |  |  |  | 3 years |  |  |
Long-Term Shareholder Value (LTSV) 2014 [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Shares vested |  |  |  |  | 116,989 |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] |  |  |  |  |  |  |
Vested (shares) |  |  |  |  | (116,989) |  |
PRSP 2012 [Member] | Restricted Stock [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Shares vested |  | 166,929 |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] |  |  |  |  |  |  |
Vested (shares) |  | (166,929) |  |  |  |  |
Forfeited (shares) |  |  |  |  |  |  | (171,083)
PRSP 2013 [Member] | Restricted Stock [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] |  |  |  |  |  |  |
Forfeited (shares) |  |  |  |  |  | (244,899) |
PRSP 2014 [Member] | Restricted Stock [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] |  |  |  |  |  |  |
Forfeited (shares) |  |  |  |  | (214,571) |  |
PRSP 2015 [Member] | Restricted Stock [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Shares vested |  |  |  | 126,585 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] |  |  |  |  |  |  |
Vested (shares) |  |  |  | (126,585) |  |  |
Forfeited (shares) |  |  |  | (196,196) |  |  |
PSU [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Adjustment percentage |  |  |  | 20.00% |  |  |
PSU [Member] | Performance Awards [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Income target attainment measurement period |  |  |  | 3 years |  |  |
Performance measurement period |  |  |  | 3 years |  |  |
Share based compensation expense | $ |  |  |  | $ 3.8 | $ 1.9 |  |
Common stock shares available for future awards |  |  |  | 2,200,000 |  |  |
Unrecognized compensation expense | $ |  |  |  | $ 8.8 |  |  |
TSR and TSRP Awards [Member] | Performance Awards [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Performance measurement period |  |  |  | 3 years |  |  |
Share based compensation expense | $ |  |  |  | $ 3.6 | $ 6.6 | $ 10.6 |
2016 PSU [Member] | Performance Awards [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Granted (shares) |  |  |  |  | 936,558 |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] |  |  |  |  |  |  |
Granted (shares) |  |  |  |  | 936,558 |  |
2017 PSU [Member] | Performance Awards [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Granted (shares) |  |  |  | 589,612 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] |  |  |  |  |  |  |
Granted (shares) |  |  |  | 589,612 |  |  |
Minimum [Member] | Restricted Stock [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Award vesting period |  |  |  | 3 years |  |  |
Minimum [Member] | PSU [Member] | Performance Awards [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Expected compensation cost | $ |  |  |  | $ 0.0 |  |  |
Attainment level range (percent) |  |  |  | 25.00% |  |  |
Minimum [Member] | TSR and TSRP Awards [Member] | Performance Awards [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Award multiplier |  |  |  | 0 |  |  |
Maximum [Member] | Restricted Stock [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Award vesting period |  |  |  | 5 years |  |  |
Maximum [Member] | PSU [Member] | Performance Awards [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Expected compensation cost | $ |  |  |  | $ 33.3 |  |  |
Attainment level range (percent) |  |  |  | 150.00% |  |  |
Maximum [Member] | TSR and TSRP Awards [Member] | Performance Awards [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Award multiplier |  |  |  | 2 |  |  |
Forecast [Member] | Restricted stock and RSUs [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Share based compensation expense | $ |  |  | $ 4.6 |  |  |  |
Forecast [Member] | PSU [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Share based compensation expense | $ |  |  | $ 4.9 |  |  |  |
Allegheny Technologies $350 million 9.375% Senior Notes due 2019 [Member] |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Debt instrument carrying amount | $ |  |  |  | $ 0.0 | $ 350.0 |  |
Debt instrument interest rate stated percentage |  |  |  |  | 9.375% |  |

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- Definition The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(2) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(2) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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