Summary of Significant Accounting Policies Accumulated other comprehensive income (loss) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Net Periodic Benefit to be Recognized in Next Fiscal Year	$ 3.2
Deferred Hedging Losses to be Recognized	4.9
Accumulated other comprehensive loss, net of tax	(40.6)	(60.2)
Atoka Noncontrolling Interest [Member]
Accumulated other comprehensive loss, net of tax	0	0
Defined Benefit Plans Income Loss [Member] | Pension Plans, Defined Benefit [Member]
Accumulated Other Comprehensive Income (Loss) Pension and Other Postretirement Plans, Net of Tax	2.7
Defined Benefit Plans Income Loss [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Accumulated Other Comprehensive Income (Loss) Pension and Other Postretirement Plans, Net of Tax	1.9
Defined Benefit Plan Prior Service Cost [Member] | Pension Plans, Defined Benefit [Member]
Accumulated Other Comprehensive Income (Loss) Pension and Other Postretirement Plans, Net of Tax	0.3
Defined Benefit Plan Prior Service Cost [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Accumulated Other Comprehensive Income (Loss) Pension and Other Postretirement Plans, Net of Tax	(1.8)
Defined Benefit Plan Net Transition Obligation [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Accumulated Other Comprehensive Income (Loss) Pension and Other Postretirement Plans, Net of Tax	0.1
Defined Benefit Plans Income Loss [Member] | Pension Plans, Defined Benefit [Member]
Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax	(42.1)	(31.1)
Defined Benefit Plans Income Loss [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax	(15.4)	(13.6)
Defined Benefit Plan Prior Service Cost [Member] | Pension Plans, Defined Benefit [Member]
Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax	(0.1)	(0.5)
Defined Benefit Plan Prior Service Cost [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax	9.0	0
Defined Benefit Plan Net Transition Obligation [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax	(0.1)	(0.3)
Commodity Contract [Member]
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax	3.3	(19.5)
Interest Rate Swap [Member]
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax	(0.7)	(1.0)
Parent [Member]
Accumulated other comprehensive loss, net of tax	(46.1)	(66.0)
Noncontrolling Interest [Member]
Accumulated other comprehensive loss, net of tax	$ (5.5)	$ (5.8)