Benefit Plans (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Pension and Other Postretirement Benefit Expense [Abstract]
Pension cost	$ 11.9	$ 22.3	$ 20.3
Other postretirement benefit (income) expense	(13.7)	(4.2)	2.5
Defined contribution plan, cost recognized	6.4	6.0	7.2
Regulatory Assets [Line Items]
Regulatory assets	316.9	294.1
Regulatory Liabilities [Line Items]
Regulatory liabilities	333.1	287.6
Salaries, Wages and Officers' Compensation [Abstract]
Allocated share-based compensation expense	3.0	3.0	2.8
Other postretirement benefits [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	39.1	25.3
Regulatory liability, deferred for future rate treatment	23.0	6.6
Regulatory liability, being amortized	$ 16.1	$ 18.7