Commitments and Contingencies (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	24 Months Ended	27 Months Ended	39 Months Ended
	Mar. 31, 2015	Dec. 31, 2014 case	Mar. 31, 2015 case	Mar. 31, 2015	Oct. 09, 2014 case
Commitments and Contingencies
Number of heavy equipment statutes cases tried and completed in Texas state district court		3
Number of heavy equipment statutes cases appealed			3
Number of district court rulings for which oral arguments were made					2
Ad valorem tax benefit	$ 3.1			$ 23.8
Ad valorem tax benefit agreed to by a number of appraisal review boards and county appraisal districts	4.6		4.6	4.6
Ad valorem tax benefit in litigation	19.2		19.2	19.2
Accrued liability for the outcomes of non-income based tax audits	$ 0.1	$ 0.1	$ 0.1	$ 0.1