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Income Taxes (Details) $ in Millions, ₩ in Billions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2019 USD ($) | Mar. 31, 2018 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2018 KRW (₩)
Income Taxes [Line Items] |  |  |  |
Income tax provision | $ 14.0 | $ 7.3 |  |
Effective income tax rate | 27.30% | 19.00% |  |
Adjustment for Income Tax Benefit, prior year foreign tax return | $ 2.8 |  |  |
Adjustment for Income Tax Benefit, changes in estimates in uncertain tax positions, an adjustment for stock-based compensation, and changes in valuation allowance |  | $ 4.9 |  |
Tax Authority, South Korea (SRTO) |  |  |  |
Income Taxes [Line Items] |  |  |  |
Income tax assessment notice, amount |  |  | $ 9.9 | ₩ 11.3
Pro forma |  |  |  |
Income Taxes [Line Items] |  |  |  |
Effective income tax rate | 32.80% | 31.70% |  |
Adjustment for Income Tax Benefit, prior year foreign tax return | $ 3.2 |  |  |

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