Pension and Other Postretirement Benefits - Benefit Costs (Details 5) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended			1 Months Ended	36 Months Ended
	Dec. 31, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2012	Dec. 31, 2013
Recognized net actuarial loss (gain):
Pension corridor charge	$ 2.0	$ 2.0	$ 0	$ 157.3
Expense for defined contribution plans		12.1	11.5	11.6
Pension Plans, Defined Benefit [Member]
Components of pension and OPEB expense (income):
Service cost		1.7	2.4	3.2
Interest cost		146.0	138.8	160.2
Expected return on plan assets		(202.8)	(184.5)	(188.3)
Amortization of prior service cost (credit)		4.3	3.8	3.8
Reversal of prior amortization related to Zanesville Retiree Settlement		0	0	0
Recognized net actuarial loss (gain):
Annual amortization		2.5	23.6	24.5
Pension corridor charge		2.0	0	157.3
Settlement (gain) loss		0.2	(0.8)	0
Pension and OPEB expense (income)		(46.1)	(16.7)	160.7
Increase in benefit obligation from litigation settlement		2.0	6.0
Estimated net loss (gain) expected to be amortized into pension and OPEB expense (income) in the next year		31.3
Estimated net prior service cost (credit) expected to be amortized into pension and OPEB expense (income) in the next year		4.4
Other Postretirement Benefit Plans, Defined Benefit [Member]
Components of pension and OPEB expense (income):
Service cost		4.9	4.7	4.6
Interest cost		21.7	21.0	31.5
Expected return on plan assets		0	0	0
Amortization of prior service cost (credit)		(73.2)	(80.0)	(77.4)
Reversal of prior amortization related to Zanesville Retiree Settlement		0	0
Recognized net actuarial loss (gain):
Annual amortization		(1.3)	2.4	0.1
Pension corridor charge		0	0	0
Settlement (gain) loss		3.5	0	0
Pension and OPEB expense (income)		(44.4)	(51.9)	(38.7)
Increase in benefit obligation from litigation settlement		(12.8)	0
Estimated net loss (gain) expected to be amortized into pension and OPEB expense (income) in the next year		1.6
Estimated net prior service cost (credit) expected to be amortized into pension and OPEB expense (income) in the next year		(64.7)
Zanesville Works Retiree Healthcare Benefits Litigation [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Components of pension and OPEB expense (income):
Reversal of prior amortization related to Zanesville Retiree Settlement				2.5
Recognized net actuarial loss (gain):
Amount of combined lump sum payments to VEBA and plaintiff's counsel					10.6
Increase in benefit obligation from litigation settlement				3.0
Reversal of Prior Amortization Related to Retiree Settlement and Legal Fees				3.8
Butler Works Retiree Healthcare Benefits Litigation [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Recognized net actuarial loss (gain):
Payments for Legal Settlements						$ 91.0