Summary of Significant Accounting Policies (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Percentage of customer revenue	43.00%	43.00%	45.00%
Accounts receivable recorded investment past due days	90
Self insurance retention liability	$ 3.0	$ 2.0	$ 2.0
Self insurance retention liability additional deductible	5.0
Self insurance retention Workers' compensation	1.0
Letters of credit outstanding, amount	39.6
Cumulative effect on retained earnings	0.3
State of Nebraska [Member]
Workers' compensation insurance bonds	26.6
Workers' Compensation Insurance Carrier [Member]
Workers' compensation insurance bonds	6.9
Minimum
Self insurance retention liability additional deductible applicable range	5.0
Maximum
Self insurance retention liability additional deductible applicable range	$ 10.0