Other Comprehensive Income (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Defined benefit pension plan and VEBAs, before tax
Net actuarial (loss) gain arising during the period, before tax	$ 87.8	$ (110.6)	$ 25.5
Reclassification adjustments:
Less: amortization of net actuarial loss, before tax	3.1	0.6	0.7
Less: amortization of prior service cost, before tax	4.2	4.2	4.2
Other comprehensive income (loss) relating to defined benefit pension plan and VEBAs	95.1	(105.8)	30.4
Unrealized gain (loss) on available for sale securities, before tax	0.6	(0.1)	0.1
Foreign currency translation adjustment, before tax	(0.2)	0.2	(0.5)
Other comprehensive income (loss), before tax	95.5	(105.7)	30.0
Defined benefit pension plan and VEBAs, Income tax (expense) benefit
Net actuarial (gain) loss arising during the period, income tax (expense) benefit	(33.5)	42.2	(9.7)
Reclassification adjustments, Income tax expense:
Less: amortization of net actuarial loss, income tax expense	(1.1)	(0.2)	(0.2)
Less: amortization of prior service cost, income tax expense	(1.7)	(1.6)	(1.6)
Total income recognized in Accumulated other comprehensive income related to defined benefit pension plan and VEBAs, Income tax expense	(36.3)	40.4	(11.5)
Unrealized gain on available for sale securities, Income tax expense	(0.2)	0	0
Foreign currency translation adjustment, Income tax expense	0	0	0
Income tax (expense) benefit related to items of other comprehensive income (loss)	(36.5)	40.4	(11.5)
Defined benefit pension plan and VEBAs, net of tax
Net actuarial (loss) gain arising during the period, net-of-tax	54.3	(68.4)	15.8
Reclassification Adjustments, Net of Tax:
Less: amortization of net actuarial loss, net of tax	2.0	0.4	0.5
Less: amortization of prior service cost, net of tax	2.5	2.6	2.6
Total income recognized in Accumulated other comprehensive income related to defined benefit pension plan and VEBAs, net of tax	58.8	(65.4)	18.9
Unrealized gain on available for sale securities, net of tax	0.4	(0.1)	0.1
Foreign currency translation adjustment, net of tax	(0.2)	0.2	(0.5)
Other comprehensive income (loss), net of tax	$ 59.0	$ (65.3)	$ 18.5