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Stockholders' Equity (Details) $ / shares in Units, $ in Millions |  | 1 Months Ended | 12 Months Ended
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Nov. 07, 2017 $ / shares shares | Feb. 28, 2018 shares | Feb. 28, 2017 shares | Dec. 31, 2019 USD ($) $ / shares shares | Dec. 31, 2018 USD ($) $ / shares shares | Dec. 31, 2017 USD ($) $ / shares shares | Dec. 31, 2016 USD ($) $ / shares shares | Dec. 31, 2015 $ / shares shares | Dec. 31, 2014 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 | $ |  |  |  |  | $ 0.0 | $ 397.8 | $ 0.0 |  |
Granted (shares) |  |  |  |  | 290,000 | 378,000 | 682,000 |  |
Common stock shares available for future awards |  |  |  |  | 5,300,000 |  |  |  |
Shares vested |  |  |  |  | 540,000 | 591,000 | 154,000 |  |
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) |  |  |  | 1,055,000 | 1,320,000 | 1,852,000 | 1,652,000 |  |
Granted (shares) |  |  |  |  | 290,000 | 378,000 | 682,000 |  |
Vested (shares) |  |  |  |  | (540,000) | (591,000) | (154,000) |  |
Forfeited (shares) |  |  |  |  | (15,000) | (319,000) | (328,000) |  |
Nonvested end of the year (shares) |  |  |  |  | 1,055,000 | 1,320,000 | 1,852,000 | 1,652,000 |
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 |  |  |  | $ 28,300,000 | $ 32,500,000 | $ 51,500,000 | $ 57,000,000 |  |
Granted (in dollars per share) | $ / shares |  |  |  |  | 7,800,000 | 7,100,000 | 8,400,000 |  |
Vested (in dollars per share) | $ / shares |  |  |  |  | (11,700,000) | (16,900,000) | (4,300,000) |  |
Forfeited (in dollars per share) | $ / shares |  |  |  |  | (300,000) | (9,200,000) | (9,600,000) |  |
Ending balance (in dollars per share) | $ / shares |  |  |  |  | $ 28,300,000 | $ 32,500,000 | $ 51,500,000 | $ 57,000,000 |
Retained Earnings Note Disclosure [Abstract] |  |  |  |  |  |  |  |  |
Undistributed earnings of investees accounted for under equity method | $ |  |  |  |  | $ 24.0 |  |  |  |
Restricted Stock [Member] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Income target attainment measurement period |  |  |  |  | 3 years |  |  |  |
Restricted stock and RSUs [Member] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Share based compensation expense | $ |  |  |  |  | $ 9.7 | $ 9.8 | $ 11.2 |  |
Unrecognized compensation expense | $ |  |  |  |  | $ 5.3 |  |  |  |
Performance Awards [Member] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] |  |  |  |  |  |  |  |  |
Forfeited (shares) |  |  |  |  | (21,848) | (67,521) | (17,598) |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward] |  |  |  |  |  |  |  |  |
Granted (in dollars per share) | $ / shares |  |  |  |  | $ 12,900,000 | $ 12,400,000 | $ 11,500,000 |  |
Forfeited (in dollars per share) | $ / shares |  |  |  |  | $ (500,000) | $ (1,000,000) | $ (200,000) |  |
2016 LTIP [Member] | Restricted Share Units [Member] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Granted (shares) |  |  |  |  |  |  | 587,661 |  |
Shares vested |  | 178,335 | 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) |  | (178,335) | (190,421) |  |  |  |  |  |
2017 RSU [Member] | Restricted Share Units [Member] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Granted (shares) |  |  |  |  |  | 320,808 |  |  |
Shares vested |  | 102,279 |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] |  |  |  |  |  |  |  |  |
Granted (shares) |  |  |  |  |  | 320,808 |  |  |
Vested (shares) |  | (102,279) |  |  |  |  |  |  |
2018 RSU [Member] | Restricted Share Units [Member] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Granted (shares) |  |  |  |  | 253,393 |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] |  |  |  |  |  |  |  |  |
Granted (shares) |  |  |  |  | 253,393 |  |  |  |
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 |  |  |  |  |  | 73,734 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] |  |  |  |  |  |  |  |  |
Vested (shares) |  |  |  |  |  | (73,734) |  |  |
Forfeited (shares) |  |  |  |  |  | (49,148) |  |  |
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 [Line Items] |  |  |  |  |  |  |  |  |
Shares vested |  | 233,896 |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] |  |  |  |  |  |  |  |  |
Vested (shares) |  | (233,896) |  |  |  |  |  |  |
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 | $ |  |  |  |  | $ 11.4 | $ 3.8 | $ 1.9 |  |
Common stock shares available for future awards |  |  |  |  | 1,700,000 |  |  |  |
Unrecognized compensation expense | $ |  |  |  |  | $ 12.1 |  |  |  |
TSR Modifier [Member] | Performance Awards [Member] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Performance measurement period |  |  |  |  |  | 3 years |  |  |
TSR and TSRP Awards [Member] | Performance Awards [Member] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Share based compensation expense | $ |  |  |  |  |  | $ 3.6 | $ 6.6 |  |
2016 PSU [Member] | Performance Awards [Member] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Granted (shares) |  |  |  |  |  |  | 936,558 |  |
Shares vested |  |  |  |  | 329,897 |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] |  |  |  |  |  |  |  |  |
Granted (shares) |  |  |  |  |  |  | 936,558 |  |
Vested (shares) |  |  |  |  | (329,897) |  |  |  |
2017 PSU [Member] | Performance Awards [Member] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Granted (shares) |  |  |  |  |  | 589,980 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] |  |  |  |  |  |  |  |  |
Granted (shares) |  |  |  |  |  | 589,980 |  |  |
2018 PSU [Member] | Performance Awards [Member] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Granted (shares) |  |  |  |  | 456,318 |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] |  |  |  |  |  |  |  |  |
Granted (shares) |  |  |  |  | 456,318 |  |  |  |
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 |  |  |  |
Minimum [Member] | TSR and TSRP Awards [Member] | Performance Awards [Member] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Award multiplier |  |  |  |  |  | 0 |  |  |
Minimum [Member] | 2018 PSU [Member] | Performance Awards [Member] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Attainment level range (percent) |  |  |  |  | 25.00% |  |  |  |
Minimum [Member] | 2016 and 2017 PSU [Member] | Performance Awards [Member] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Attainment level range (percent) |  |  |  |  | 25.00% |  |  |  |
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 | $ |  |  |  |  | $ 34.8 |  |  |  |
Maximum [Member] | TSR and TSRP Awards [Member] | Performance Awards [Member] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Award multiplier |  |  |  |  |  | 2 |  |  |
Maximum [Member] | 2018 PSU [Member] | Performance Awards [Member] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Attainment level range (percent) |  |  |  |  | 200.00% |  |  |  |
Maximum [Member] | 2016 and 2017 PSU [Member] | Performance Awards [Member] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Attainment level range (percent) |  |  |  |  | 150.00% |  |  |  |
Forecast [Member] | Restricted stock and RSUs [Member] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Share based compensation expense | $ |  |  |  | $ 4.5 |  |  |  |  |
Forecast [Member] | PSU [Member] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Share based compensation expense | $ |  |  |  | $ 9.6 |  |  |  |  |
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 | $ |  |  |  |  |  |  | $ 350.0 |  |
Debt instrument interest rate stated percentage |  |  |  |  |  |  | 9.375% |  |

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