Workers' Compensation Expense (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Reimbursable liabilities	$ 20.3
Insurance receivable, current	3.3	$ 0.8
Insurance receivable, noncurrent	$ 17.0
Traumatic and other workers’ compensation claims
Defined Benefit Plan Disclosure [Line Items]
Risk-free interest rate	2.43%
Surety bonds and letters of credit outstanding	$ 123.0