Benefit Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension and Other Postretirement Benefit Expense [Abstract]
Pension cost	$ 13.5	$ 11.9	$ 22.3
Other postretirement benefit (income) expense	(11.9)	(13.7)	(4.2)
Defined contribution plan, cost recognized	6.6	6.4	6.0
Regulatory Liabilities [Line Items]
Regulatory liabilities	385.9	333.1
Salaries, Wages and Officers' Compensation [Abstract]
Allocated share-based compensation expense	4.0	3.0	$ 3.0
Other postretirement benefits [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	51.0	39.1
Regulatory liability, deferred for future rate treatment	37.4	23.0
Regulatory liability, being amortized	13.6	16.1
Pension Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	8.0	0.0
Regulatory liability, deferred for future rate treatment	$ 8.0	0.0
Pension Costs [Member]
Regulatory Assets [Line Items]
Regulatory asset, deferred for future rate treatment		$ 0.0