Long-Term Obligations (Details) (Equipment Financing Arrangement, USD $) In Millions, unless otherwise specified	1 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Dec. 31, 2012
Long-term obligations
Draw down period			3 years
Interest rate swap period, used to calculate interest			3 years
Basis for variable interest rate			three year interest rate swap
Additional principal amount drawn during the period	$ 2.5	$ 9.1
Interest rate on new draw down (as a percent)	4.38%	4.12%	4.38%
Weighted average interest rate (as a percent)	4.54%		4.54%
Maximum borrowing capacity for equipment purchases	3.4		3.4
Total	$ 9.9		$ 9.9	$ 10.0
Minimum
Long-term obligations
Percentage added to variable rate basis	3.50%		3.50%
Maximum
Long-term obligations
Percentage added to variable rate basis	4.00%		4.00%