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Income Tax Matters, Narrative (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017 | Dec. 31, 2017
Income Tax Disclosure [Abstract] |  |  |  |  |
Income tax provision | $ 7.8 | $ 2.2 | $ 13.7 | $ 20.7 |
Effective tax rate (percent) | 27.50% | 31.70% | 22.80% | 33.80% |
Effective Income Tax Rate Reconciliation, Share-based Compensation, Excess Tax Benefit, Amount | $ (0.3) | $ (0.1) | $ (2.0) | $ (1.7) |
Effective Income Tax Rate Reconciliation, Share-based Compensation, Excess Tax Benefit, Percent | (0.90%) | (1.50%) | (3.40%) | (2.70%) |
Effective tax rate reconciliation, increase (decrease) to the valuation allowance for certain state net operating losses | $ 0.3 | $ (0.1) | $ (0.5) | $ (0.5) |
Effective tax rate reconciliation, increase (decrease) to the valuation allowance for certain state net operating losses (percent) | 1.20% | (1.40%) | (0.80%) | (0.90%) |
Effective Income Tax Rate Reconciliation, Unrecognized Tax Benefit Amount |  | $ (0.2) |  | $ (0.2) |
Effective Income Tax Rate Reconciliation Unrecognized Tax Benefit Percent |  | (3.00%) |  | (0.30%) |
Effective Income Tax Rate Reconciliation Sequestration of AMT Credits Amount | $ 0.2 |  | $ 0.4 |  |
Effective Income Tax Rate Reconciliation Sequestration of AMT Credits Percent | 0.70% |  | 0.60% |  |
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Amount | $ 0.6 |  | $ 1.2 |  |
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Percent | 2.10% |  | 2.00% |  |
Gross unrecognized tax benefits | $ 1.5 |  | $ 1.5 |  | $ 1.5
Gross unrecognized tax benefits that would impact effective tax rate | $ 0.4 |  | $ 0.4 |  | $ 0.4

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