Income Tax Expense - Deferred Income Tax Assets and Liabilities (Details) - USD ($) $ in Millions |
Dec. 31, 2019 |
Dec. 31, 2018 |
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|---|---|---|---|---|
| Deferred Income Tax Assets [Abstract] | ||||
| Employee Benefits and Compensation | $ 49.9 | $ 62.2 | ||
| Property Related | 76.9 | 95.2 | ||
| NOL Carryforwards | 63.2 | 86.1 | ||
| Tax Credit Carryforwards | 395.5 | 349.8 | ||
| Power Sales Agreements | 23.7 | 27.5 | ||
| Regulatory Liabilities | 116.9 | 113.4 | ||
| Other | 23.4 | 25.1 | ||
| Gross Deferred Income Tax Assets | 749.5 | 759.3 | ||
| Deferred Income Tax Asset Valuation Allowance | (70.0) | (66.5) | ||
| Total Deferred Income Tax Assets | 679.5 | 692.8 | ||
| Deferred Income Tax Liabilities [Abstract] | ||||
| Property-Related | 713.4 | 752.5 | ||
| Regulatory Assets for Benefit Obligations | 54.5 | 61.0 | ||
| Unamortized Investment Tax Credits | 31.6 | 32.2 | ||
| Partnership Basis Differences | 49.4 | 40.8 | ||
| Regulatory Assets | 35.4 | 29.9 | ||
| Other | 8.0 | 0.0 | ||
| Total Deferred Income Tax Liabilities | 892.3 | 916.4 | ||
| Net Deferred Income Taxes | [1] | $ 212.8 | $ 223.6 | |
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