Statements of Consolidated Comprehensive Income (Loss) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statement of Comprehensive Income [Abstract]
Net (loss) income	$ (236.6)	$ 71.8	$ 104.8
Defined benefit pension plan and VEBAs:
Total actuarial (loss) gain and prior service costs	(6.1)	(129.5)	2.2
Reclassification adjustments:
Amortization of net actuarial loss (gain)	1.1	(1.8)	1.5
Amortization of prior service cost	3.0	10.6	4.2
Accumulated Defined Benefit Plans Adjustment, Removal Of Obligation Attributable to Parent	106.6	0.0	0.0
Other comprehensive income (loss) relating to defined benefit pension plan and VEBAs	104.6	(120.7)	7.9
Available for sale securities:
Unrealized (loss) gain on available for sale securities	(0.1)	(0.2)	1.0
Reclassification adjustments:
Reclassification of unrealized gain upon sale of available for sale securities	(0.4)	(0.1)	(1.0)
Other comprehensive loss relating to available for sale securities	(0.5)	(0.3)	0.0
Unrealized loss on foreign currency cash flow hedges	(0.3)	0.0	0.0
Foreign currency translation (loss) gain	(0.2)	0.4	0.2
Other comprehensive income (loss), before tax	103.6	(120.6)	8.1
Income tax (expense) benefit related to items of other comprehensive income (loss)	(39.2)	45.2	(2.8)
Other comprehensive income (loss), net of tax	64.4	(75.4)	5.3
Comprehensive (loss) income	$ (172.2)	$ (3.6)	$ 110.1