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Income Taxes - Income Tax Contingencies (Details) $ in Millions, ₩ in Billions | 3 Months Ended | 12 Months Ended
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Jun. 30, 2018 USD ($) | Jun. 30, 2018 KRW (₩) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Income Tax Disclosure [Abstract] |  |  |  |  |
Increase (decrease) in additional liabilities related to unrecognized tax liabilities |  |  | $ (1.8) | $ 3.4 |
Increase (decrease) in assets related to unrecognized tax assets |  |  | 4.3 | 19.8 |
Increase (decrease) in unrecognized tax liabilities related to a foreign currency translation expense |  |  | 0.9 | (1.4) |
Unrecognized tax liabilities |  |  | 70.2 | 72.6 |
Unrecognized tax assets |  |  | 29.7 | 25.4 |
Reconciliation of total amounts of unrecognized income tax benefits |  |  |  |  |
Gross Unrecognized Tax Liabilities – opening balance |  |  | 58.8 | 62.2 | $ 47.8
Gross increases – tax positions in prior period |  |  | 3.6 | 10.9 | 19.7
Gross decreases – tax positions in prior period |  |  | (10.6) | 0.0 | (15.4)
Gross increases – tax positions in current period |  |  | 11.5 | 10.7 | 12.9
Gross decreases – tax positions in current period |  |  | 0.0 | 0.0 | 0.0
Settlements |  |  | (1.5) | (23.0) | 0.0
Lapse of statute of limitations |  |  | (4.8) | (2.1) | (2.8)
Gross Unrecognized Tax Liabilities – ending balance |  |  | 57.0 | 58.8 | 62.2
Expected reduction in income tax expense if uncertain tax positions are settled by the taxing authorities in the entity's favor |  |  | $ 27.4 |  |
Expected reduction in effective income tax rate if uncertain tax positions are settled by the taxing authorities in the entity's favor (as a percent) |  |  | 14.80% |  |
Interest and penalties expense |  |  |  |  |
Interest related to unrecognized tax benefits, recorded as income (expense) |  |  | $ (1.2) | (3.4) | 0.7
Penalties related to unrecognized tax benefits, recorded as income tax income (expense) |  |  | 1.9 | (0.1) | $ (2.3)
Accrued interest and penalties |  |  |  |  |
Accrued interest related to unrecognized tax benefits |  |  | 8.5 | 7.3 |
Accrued penalties related to unrecognized tax benefits |  |  | $ 4.6 | $ 6.5 |
South Koria (SRTO) |  |  |  |  |
Income Tax Contingency [Line Items] |  |  |  |  |
Pre-assessment notice, amount | $ 10.2 | ₩ 11.3 |  |  |

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