Summary of Significant Accounting Policies (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Percentage Of Customer Revenue	40.00%	40.00%	40.00%
Accounts receivable recorded investment past due days	90
Liability Self-insurance Retention And Deductible Amount	$ 2.0	$ 2.0	$ 2.0
Self Insurance Retention Workers Compensation	1.0
Letters of Credit Outstanding, Amount	33.8
State Of Nebraska [Member]
Workers Compensation Insurance Bonds	32.2
Workers Compensation Insurance Carrier [Member]
Workers Compensation Insurance Bonds	$ 6.9