Summary of Significant Accounting Policies (Self Insurance Retention Liability) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Self insurance Retention Liability [Line Items]
Self insurance retention liability	$ 10.0	$ 3.0
Self insurance retention liability additional deductible		5.0
Self insurance retention Workers' compensation	2.0	1.0
Letters of credit outstanding, amount	50.9
Minimum
Self insurance Retention Liability [Line Items]
Self insurance retention liability additional deductible applicable range		5.0
Maximum
Self insurance Retention Liability [Line Items]
Self insurance retention liability additional deductible applicable range		$ 10.0
State of Nebraska [Member]
Self insurance Retention Liability [Line Items]
Workers' compensation insurance bonds	25.5
Workers compensation insurance carrier [Member]
Self insurance Retention Liability [Line Items]
Workers' compensation insurance bonds	13.4
Insurance carrier [Member]
Self insurance Retention Liability [Line Items]
Letters of credit outstanding, amount	$ 39.0