Accident and Workers' Compensation Insurance Liabilities (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Accident and Workers' Compensation Insurance Liabilities
Auto liability retention limit	$ 2.0
Excess Insurance Coverage Limit	55.0
Workers compensation retention limit	1.0
Trust fund requirements	0.7
Trust fund actual deposits	1.3
Letters of Credit Outstanding, Amount	$ 3.2