Other Comprehensive Income (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Reclassification adjustments, before tax:
Amortization of net actuarial (gain) loss, before tax	$ 0.3	$ (0.3)
Amortization of prior service cost, before tax	0.7	2.8
Other comprehensive income relating to VEBAs, before tax	(107.6)	(2.5)
Available for sale securities:
Unrealized gain on available for sale securities, before tax	0	(0.1)
Reclassification of unrealized gain upon sale of available for sale securities, before tax	0.1	(0.1)
Other comprehensive income (loss) relating to available for sale securities	0.1	0
Foreign currency translation adjustment, before tax	(0.1)	(0.2)
Other comprehensive income, before tax	107.8	2.7
Reclassification adjustments, income tax (expense) benefit:
Amortization of net actuarial (gain) loss, income tax (expense) benefit	(0.1)	0.1
Amortization of prior service cost, income tax (expense) benefit	(0.3)	(1.0)
Other comprehensive income relating to VEBAs, income tax (expense) benefit	(40.8)	(0.9)
Available for sale securities:
Unrealized gain on available for sale securities, Income tax (expense) benefit		0
Reclassification adjustments, income tax (expense) benefit:
Reclassification of unrealized gain upon sale of available for sale securities, income tax (expense) benefit	(0.1)	0
Other comprehensive income (loss) relating to available for sale securities, income tax (expense) benefit	(0.1)	0
Foreign currency translation adjustment, income tax (expense) benefit	0	0
Other comprehensive income, Income tax (expense) benefit	(40.9)	(1.8)
Reclassification adjustments, net of tax:
Amortization of net actuarial (gain) loss, net of tax	0.2	(0.2)
Amortization of prior service cost, net of tax	0.4	1.8
Other comprehensive income relating to VEBAs, net of tax	66.8	1.6
Available for sale securities:
Unrealized gain on available for sale securities, net of tax		0.1
Reclassification adjustments, net of tax:
Reclassification of unrealized gain upon sale of available for sale securities, net of tax	0	(0.1)
Other comprehensive income (loss) relating to available for sale securities, net of tax	0	0
Foreign currency translation adjustment, net of tax	0.1	0.2
Other comprehensive income, net of tax	66.9	0.9
Other Comprehensive Income Cumulative Tax Rate Adjustments Before Tax		0
Other Comprehensive Income Cumulative Tax Rate Adjustments Tax		0.9
Other Comprehensive Income Cumulative Tax Rate Adjustments Net of Tax		(0.9)
Other Comprehensive Income (Loss), Finalization of Pension and Other Postretirement Benefit Plan Valuation, before Tax	106.6	0
Other Comprehensive Income (Loss), Finalization of Pension and Other Postretirement Benefit Plan Valuation, Tax	(40.4)
Other Comprehensive Income (Loss), Finalization of Pension and Other Postretirement Benefit Plan Valuation, Net of Tax	$ 66.2