Benefit Plans (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Pension and Other Postretirement Benefit Expense [Abstract]
Pension cost	$ 22.3	$ 20.3	$ 16.4
Other postretirement benefit (income) expense	(4.2)	2.5	(3.0)
Defined contribution plan, cost recognized	6.0	7.2	6.9
Regulatory Assets [Line Items]
Regulatory assets	294.1	251.6
Regulatory Liabilities [Line Items]
Regulatory liabilities	287.6	247.5
Salaries, Wages and Officers' Compensation [Abstract]
Allocated share-based compensation expense	3.0	2.8	2.4
Other postretirement benefits [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	25.3	24.7
Regulatory liability, deferred for future rate treatment	6.6
Regulatory liability, being amortized	18.7
Other postretirement benefits [Member]
Regulatory Assets [Line Items]
Regulatory assets	$ 0	$ 4.6