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Income Tax Expense - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions | Dec. 31, 2015 | Dec. 31, 2014
--- | --- | ---
Deferred Tax Assets [Abstract] |  |  |
Employee Benefits and Compensation |  | $ 105.4 | $ 102.2
Property Related |  | 126.6 | 102.7
NOL Carryforwards |  | 186.4 | 156.5
Tax Credit Carryforwards |  | 164.8 | 95.7
Power Purchase Agreements |  | 73.0 | 51.8
Other |  | 21.8 | 17.0
Gross Deferred Tax Assets |  | 678.0 | 525.9
Deferred Tax Asset Valuation Allowance |  | (31.6) | (22.1)
Total Deferred Tax Assets |  | 646.4 | 503.8
Deferred Tax Liabilities [Abstract] |  |  |
Property Related |  | 1,053.0 | 848.8
Regulatory Asset for Benefit Obligations |  | 89.4 | 89.9
Unamortized Investment Tax Credits |  | 26.0 | 10.3
Partnership Basis Differences |  | 47.8 | 41.9
Other |  | 10.0 | 16.1
Total Deferred Tax Liabilities |  | 1,226.2 | 1,007.0
Net Deferred Income Taxes |  | 579.8 | 503.2
Net Current Deferred Tax Assets | [1] | 0.0 | 7.5
Net Long-Term Deferred Tax Liabilities |  | $ 579.8 | $ 510.7
[1] | For discussion of classification of deferred income taxes see Note 1. Operations and Significant Accounting Policies - New Accounting Standards - Balance Sheet Classification of Deferred Taxes.

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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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 45 -Paragraph 27 -URI http://asc.fasb.org/extlink&oid=37586315&loc=d3e32262-109318 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 255 -SubTopic 10 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=39738744&loc=d3e2408-110839 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.26(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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