Fair Value Measurements, Fair Value of Non financial Assets and Liabilities (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011	Dec. 31, 2010
Asset Retirement Obligation, Roll Forward
Beginning balance	$ 4.0		$ 3.8	$ 3.5
Liabilities incurred during the period	0		0	0
Liabilities settled during the period	(0.5)		(0.1)	0
Accretion expense	0.3		0.3	0.3
Adjustment to accretion expense due to revisions to estimated cash flow and timing of expenditure	0.3	[1]	0	(1.1)	[1]
Adjustment to CARO asset due to revisions to estimated cash flow	0		0	1.1
Ending balance	$ 4.1		$ 4.0	$ 3.8
(Decrease)/increase in basic earnings per share resulting from adjustment	$ (0.02)			$ 0.05
(Decrease)/increase in diluted earnings per share resulting from adjustment	$ (0.02)			$ 0.05
Weighted-average credit-adjusted risk-free rate	8.70%		9.10%

[1] The adjustment in 2012 decreased both basic and diluted earnings per share for 2012 by approximately $0.02 per share. The adjustment in 2010 increased both basic and diluted earnings per share for 2010 by approximately $0.05 per share.