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Income Taxes (Details) $ in Millions, ₩ in Billions | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2019 USD ($) | Sep. 30, 2018 USD ($) | Sep. 30, 2019 USD ($) | Sep. 30, 2018 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2016 KRW (₩)
Income Taxes [Line Items] |  |  |  |  |  |
Income tax provision | $ 21.5 | $ 23.0 | $ 55.5 | $ 42.7 |  |
Effective income tax rate | 30.30% | 37.30% | 30.90% | 30.10% |  |
Discrete tax benefit, closing US tax audit and return to provision adjustments, and recording of a valuation allowance | $ 1.3 |  |  |  |  |
Adjustment for income tax expense, change in valuation allowance in the U.S. |  | $ 1.5 |  |  |  |
Adjustment for income tax benefit, amounts that should have been recognized in a prior period, closure of US tax audit and recording of a valuation allowance |  |  | $ 3.3 |  |  |
Adjustment for income tax benefit, adjustment pursuant to the Tax Reform Act, changes in valuation allowance and adjustment for stock-based compensation |  |  |  | $ 7.4 |  |
Tax Authority, South Korea (SRTO) |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |
Income tax assessment notice, amount |  |  |  |  | $ 9.4 | ₩ 11.3
Pro forma |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |
Effective income tax rate | 32.20% | 34.90% | 32.70% | 35.30% |  |

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