Basis and Summary of Significant Accounting Policies - Income Taxes (Details) - USD ($) $ in Millions |
9 Months Ended | 12 Months Ended |
|---|---|---|
Sep. 30, 2018 |
Dec. 31, 2017 |
|
| Organization, Consolidation and Presentation of Financial Statements [Abstract] | ||
| Tax Cuts And Jobs Act Of 2017, Income tax expense (benefit) | $ (2.3) | $ (140.0) |