Restricted Cash (Narrative) (Details)	1 Months Ended
	Apr. 30, 2018 USD ($) item	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Restricted Cash And Cash Equivalents Items [Line Items]
Surety, payment and performance bonds		$ 39,600,000	$ 39,500,000
Collateral payment for performance bonds		0	0
Collateral related to workers compensation policy		2,500,000	$ 2,500,000
Subsequent Event [Member]
Restricted Cash And Cash Equivalents Items [Line Items]
Number of acquisitions closed | item	1
Escrow disbursement for acquisition	$ 10,000,000
Cash Held in Escrow for Acquisitions [Member] | Other Assets - Noncurrent [Member]
Restricted Cash And Cash Equivalents Items [Line Items]
Restricted cash - noncurrent asset		$ 24,502,000