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Recent Authoritative Accounting Guidance - New Accounting Pronouncements and Changes in Accounting Principles (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Reduction in income tax expense | $ 2.6 | $ 0.0 | $ 0.0
Stock Option Awards |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Reduction in income tax expense | 2.0 |  |
Restricted Stock |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Reduction in income tax expense | 0.6 |  |
Accounting Standards Update 2018-02 | Adjustments for New Accounting Principle, Early Adoption |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Stranded amounts resulting from remeasurement of deferred tax assets and liabilities | $ 3.1 |  |

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