Derivative Instruments and Hedging Activities (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013 gal	Jun. 30, 2012	Jun. 30, 2013 agreement gal	Jun. 30, 2012	Dec. 31, 2012
Derivative Instruments and Hedging Activities
Payments made or received under hedge agreements				$ 0
Fuel hedge
Derivative Instruments and Hedging Activities
Number of master agreements			1
Total gallons of diesel fuel	4,300,000		4,300,000
Gallons of diesel fuel as a percentage of total estimated annual usage	28.60%		28.60%
Fair value of derivative asset	0.3		0.3		0.6
Net receipts from the counterparty	0.1	0.4	0.3	0.8
Period over which deferred gain (loss) is expected to be reclassified from AOCI			12 months
Amount of deferred gain (loss) expected to be reclassified from AOCI			0.2
Fuel hedge | Minimum
Derivative Instruments and Hedging Activities
Diesel fuel price (in dollars per gallon)	3.63		3.63
Fuel hedge | Maximum
Derivative Instruments and Hedging Activities
Diesel fuel price (in dollars per gallon)	4.02		4.02
Interest rate swap agreements
Derivative Instruments and Hedging Activities
Period over which deferred gain (loss) is expected to be reclassified from AOCI			12 months
Amount of deferred gain (loss) expected to be reclassified from AOCI			2.0
Net payments to the counterparty	0.5		1.0
Notional amount of debt obligations	400.0		400.0
Effective rate of interest of debt (as a percent)	4.49%		4.49%
Minimum variable interest rate of debt (as a percent)	1.00%		1.00%
Fair value of derivative liability	$ 3.6		$ 3.6		$ 4.6