Summary of Significant Accounting Policies (Self Insurance Retention Liability (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2016
Self insurance Retention Liability [Line Items]
Self insurance retention liability	$ 3.0	$ 2.0
Self insurance retention liability aggregate	6.0
Self insurance retention liability additional deductible	5.0
Self insurance retention Workers' compensation	1.0
Letters of credit outstanding, amount	30.3
Minimum
Self insurance Retention Liability [Line Items]
Self insurance retention aggregate deductible applicable range	3.0
Self insurance retention liability additional deductible applicable range	5.0
Maximum
Self insurance Retention Liability [Line Items]
Self insurance retention aggregate deductible applicable range	5.0
Self insurance retention liability additional deductible applicable range	10.0
State of Nebraska [Member]
Self insurance Retention Liability [Line Items]
Workers' compensation insurance bonds	26.7
Workers compensation insurance carrier [Member]
Self insurance Retention Liability [Line Items]
Workers' compensation insurance bonds	$ 6.9