Income Tax Expense - Deferred Tax Assets and Liabilities (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012
Deferred Tax Assets [Abstract]
Employee Benefits and Compensation	$ 66.3		$ 120.2
Property Related	82.2		59.8
NOL Carryforwards	112.8		90.8
Tax Credit Carryforwards	55.1		28.3
Other	16.9		24.6
Gross Deferred Tax Assets	333.3		323.7
Deferred Tax Asset Valuation Allowance	(8.0)		(2.4)
Total Deferred Tax Assets	325.3		321.3
Deferred Tax Liabilities [Abstract]
Property Related	656.2		577.1
Regulatory Asset for Benefit Obligations	58.7		104.3
Unamortized Investment Tax Credits	11.1		11.9
Partnership Basis Differences	36.7		28.6
Other	22.7		30.1
Total Deferred Tax Liabilities	785.4		752.0
Net Deferred Income Taxes	460.1		430.7
Net Current Deferred Tax Assets	19.0	[1]	0	[1]
Net Current Deferred Tax Liabilities	0	[2]	6.9	[2]
Net Long-Term Deferred Tax Liabilities	$ 479.1		$ 423.8

[1] In 2013, Current Deferred Tax Assets reflect the expectation of using federal NOL carryforward deductions in 2014.

[2] Included in Other Current Assets and Other Current Liabilities.