Statements of Consolidated Comprehensive Income (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Net income	$ 9.1	$ 29.2	$ 21.0	$ 26.5	$ 6.1	$ 4.1	$ 4.1	$ 10.8	$ 85.8	$ 25.1	$ 12.0
Defined benefit pension plan and VEBAs
Net actuarial gain (loss) arising during the period									87.8	(110.6)	25.5
Reclassification adjustments relating to VEBAs:
Amortization of net actuarial loss									3.1	0.6	0.7
Amortization of prior service cost									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									0.6	(0.1)	0.1
Foreign currency translation adjustment									(0.2)	0.2	(0.5)
Other comprehensive income (loss), before tax									95.5	(105.7)	30.0
Income tax (expense) benefit related to items of other comprehensive income (loss)									(36.5)	40.4	(11.5)
Other comprehensive income (loss), net of tax									59.0	(65.3)	18.5
Comprehensive income (loss)									$ 144.8	$ (40.2)	$ 30.5