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Income Tax Expense - Deferred Tax Assets and Liabilities (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2011
Dec. 31, 2010
Deferred Tax Assets [Abstract]    
Employee Benefits and Compensation $ 132.7 $ 121.8
Property Related 56.4 51.1
NOL and Tax Credit Carryforward 78.1 28.2
Investment Tax Credits 9.0 9.7
Other 7.2 12.7
Gross Deferred Tax Assets 283.4 223.5
Deferred Tax Asset Valuation Allowance (0.4) (0.5)
Total Deferred Tax Assets 283.0 223.0
Deferred Tax Liabilities [Abstract]    
Property Related 482.7 387.2
Regulatory Asset for Benefit Obligations 117.9 105.8
Unamortized Investment Tax Credits 12.8 13.7
Partnership Basis Differences 24.4 19.4
Other 24.0 27.3
Total Deferred Tax Liabilities 661.8 553.4
Net Deferred Income Taxes 378.8 330.4
Net Current Deferred Tax Liabilities 5.2 [1] 5.2 [1]
Net Long-Term Deferred Tax Liabilities $ 373.6 $ 325.2
[1] (a)Included in Other Current Liabilities.