Summary of Significant Accounting Policies Accumulated Other Comprehensive Income (Loss) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Accumulated Other Comprehensive Income (Loss), Net of Tax	$ (41.5)	$ (40.6)
Atoka Noncontrolling Interest [Member]
Accumulated Other Comprehensive Income (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 Benefit Plans, Net of Tax	(40.6)	(42.1)
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	(14.4)	(15.4)
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	(0.1)
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	8.1	9.0
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)	(0.1)
Commodity Contract [Member]
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax	(0.3)	3.3
Interest Rate Swap [Member]
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax	(0.6)	(0.7)
Parent [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax	(47.9)	(46.1)
Noncontrolling Interest
Accumulated Other Comprehensive Income (Loss), Net of Tax	$ (6.4)	$ (5.5)