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Income Tax Expense - Deferred Income Tax Assets and Liabilities (Details) - USD ($) $ in Millions | Dec. 31, 2022 | Dec. 31, 2021
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Deferred Income Tax Assets [Abstract] |  |  |
Employee Benefits and Compensation |  | $ 46.4 | $ 44.5
Property Related |  | 61.9 | 54.8
NOL Carryforwards |  | 16.7 | 67.5
Deferred Tax Assets, Capital Loss Carryforwards |  | 13.1 | 0.0
Tax Credit Carryforwards |  | 548.7 | 509.1
Power Sales Agreements |  | 13.7 | 16.5
Regulatory Liabilities |  | 95.5 | 101.5
Other |  | 28.1 | 18.0
Gross Deferred Income Tax Assets |  | 824.1 | 811.9
Deferred Income Tax Asset Valuation Allowance |  | (60.2) | (69.0)
Total Deferred Income Tax Assets |  | 763.9 | 742.9
Deferred Income Tax Liabilities [Abstract] |  |  |
Property-Related |  | 661.7 | 680.8
Regulatory Assets for Benefit Obligations |  | 57.7 | 54.9
Unamortized Investment Tax Credits |  | 30.0 | 30.5
Partnership Basis Differences |  | 126.0 | 106.3
Regulatory Assets |  | 10.7 | 14.3
Other |  | 28.0 | 30.0
Total Deferred Income Tax Liabilities |  | 922.0 | 924.7
Net Deferred Income Taxes | [1] | 158.1 | 181.8
Deferred Tax Liabilities - Deferred Gain |  | $ 7.9 | $ 7.9
[1] | Recorded as a net Deferred Income Tax liability on the Consolidated Balance Sheet.

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- Definition The noncurrent portion of the reserve for accumulated deferred investment tax credits as of the balance sheet date. This is the remaining investment credit, which will reduce the cost of services collected from ratepayers by a ratable portion over the investment's regulatory life. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 255 -SubTopic 10 -Section 55 -Paragraph 8 -URI https://asc.fasb.org/extlink&oid=125523840&loc=d3e2408-110839 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.26(b)) -URI https://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 45 -Paragraph 27 -URI https://asc.fasb.org/extlink&oid=123427490&loc=d3e32262-109318 + Details
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