Summary of Significant Accounting Policies Accumulated Other Comprehensive Income (Loss) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	$ 3.5
Accumulated other comprehensive loss, net of tax	(49.1)	(40.6)
Atoka Noncontrolling Interest [Member]
Accumulated other comprehensive loss, net of tax	0	0
Deferred Interest Rate Swap Hedging Losses [Member]
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	(0.2)
Deferred Commodity Contracts Hedging Gains Losses [Member]
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	0.1
Defined Benefit Plan Prior Service Cost [Member]
Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax	0.1	(0.1)
Commodity Contract [Member]
Cash flow hedging gains (losses)	0.1	3.3
Interest Rate Swap [Member]
Cash flow hedging gains (losses)	(0.5)	(0.7)
Parent [Member]
Accumulated other comprehensive loss, net of tax	(58.1)	(46.1)
Noncontrolling Interest [Member]
Accumulated other comprehensive loss, net of tax	(9.0)	(5.5)
Pension Plans, Defined Benefit [Member] | Defined Benefit Plans Income Loss [Member]
Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax	(49.3)	(42.1)
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	3.3
Pension Plans, Defined Benefit [Member] | Defined Benefit Plan Prior Service Cost [Member]
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	0.1
Other Postretirement Benefit Plans, Defined Benefit [Member] | Defined Benefit Plans Income Loss [Member]
Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax	(15.7)	(15.4)
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	2.0
Other Postretirement Benefit Plans, Defined Benefit [Member] | Defined Benefit Plan Prior Service Cost [Member]
Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax	7.2	9.0
Components of Net Periodic Benefit Costs to be Recognized in Next Fiscal Year	(1.8)
Other Postretirement Benefit Plans, Defined Benefit [Member] | Defined Benefit Plan Net Transition Obligation [Member]
Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax	$ 0	$ (0.1)