Income Tax Expense - Deferred Tax Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Deferred Tax Assets [Abstract]
Employee Benefits and Compensation	$ 120.2		$ 132.7
Property Related	59.8		56.4
NOL Carryforwards	90.8		61.7
Tax Credit Carryforwards	28.3		12.2
Other	24.6		20.4
Gross Deferred Tax Assets	323.7		283.4
Deferred Tax Asset Valuation Allowance	(2.4)		(0.4)
Total Deferred Tax Assets	321.3		283.0
Deferred Tax Liabilities [Abstract]
Property Related	577.1		482.7
Regulatory Asset for Benefit Obligations	104.3		117.9
Unamortized Investment Tax Credits	11.9		12.8
Partnership Basis Differences	28.6		24.4
Other	30.1		24.0
Total Deferred Tax Liabilities	752.0		661.8
Net Deferred Income Taxes	430.7		378.8
Net Current Deferred Tax Liabilities	6.9	[1]	5.2	[1]
Net Long-Term Deferred Tax Liabilities	$ 423.8		$ 373.6

[1] Included in Other Current Liabilities.