Benefit Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension and Other Postretirement Benefit Expense [Abstract]
Pension cost	$ 8.7	$ 13.5	$ 11.9
Other postretirement benefit (income) expense	(12.0)	(11.9)	(13.7)
Defined contribution plan, cost recognized	6.5	6.6	6.4
Regulatory Liabilities [Line Items]
Regulatory liabilities	458.5	385.9
Salaries, Wages and Officers' Compensation [Abstract]
Allocated share-based compensation expense	4.0	4.0	$ 3.0
Other postretirement benefits [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	63.0	51.0
Regulatory liability, deferred for future rate treatment	52.0	37.4
Regulatory liability, being amortized	11.0	13.6
Pension Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	21.3	8.0
Regulatory liability, deferred for future rate treatment	$ 21.3	$ 8.0