Summary of Significant Accounting Policies Accumulated Other Comprehensive Income (Loss) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended			3 Months Ended
	Mar. 31, 2013	Mar. 31, 2013 Atoka Noncontrolling Interest [Member]	Dec. 31, 2012 Atoka Noncontrolling Interest [Member]	Mar. 31, 2013 Commodity Contract [Member]	Mar. 31, 2013 Interest Rate Swap [Member]	Mar. 31, 2013 Noncontrolling Interest [Member]	Mar. 31, 2013 Other Postretirement Benefit Plans, Defined Benefit [Member] Defined Benefit Plan Income Loss [Member]	Mar. 31, 2013 Other Postretirement Benefit Plans, Defined Benefit [Member] Defined Benefit Plan Prior Service Cost [Member]	Mar. 31, 2013 Pension Plans, Defined Benefit [Member] Defined Benefit Plan Income Loss [Member]	Mar. 31, 2013 Pension Plans, Defined Benefit [Member] Defined Benefit Plan Prior Service Cost [Member]
Components of Accumulated Other Comprehensive Income [Roll Forward]
Balance at December 31, 2012							$ (15.7)	$ 7.2	$ (49.3)	$ 0.1
Amounts reclassified from accumulated other comprehensive income (loss)							0.5	(0.5)	0.9	0
Balance at March 31, 2013							(15.2)	6.7	(48.4)	0.1
Components of Accumulated Other Comprehensive Income Hedging [Roll Forward]
Balance at December 31, 2012				0.1	(0.5)
Amounts reclassified from accumulated other comprehensive income (loss)				(0.1)	0.1
Balance at March 31, 2013				0	(0.4)
Components of Accumulated Other Comprehensive Income Noncontrolling Interest [Roll Forward]
Balance at December 31, 2012	(49.1)	0	0			(9.0)
Amounts reclassified from accumulated other comprehensive income (loss)	0.8					0.1
Balance at March 31, 2013	$ (48.3)	$ 0	$ 0			$ (8.9)