Long-term Debt - Tables (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Debt Instrument [Line Items]
Long-term debt	$ 35.3		$ 84.7
Outstanding letters of credit	21.9		19.8
Amounts available to borrow	145.6	[1]	97.7	[1]
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Long-term debt	24.0		73.3
Obligations under capital leases [Member]
Debt Instrument [Line Items]
Long-term debt	$ 11.3		$ 11.4

[1] Excluding $100 million expansion feature