Subsequent Events US Short Line Tax Credit (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011
Subsequent Event [Line Items]
tax benefit from extension of the short line tax credit	$ 35.0	$ 2.2	$ 3.2	$ 2.5	$ 2.2