Restricted Cash (Narrative) (Details) - USD ($)	Mar. 31, 2019	Dec. 31, 2018
Restricted Cash And Cash Equivalents Items [Line Items]
Surety, payment and performance bonds	$ 40,600,000	$ 40,500,000
Collateral payment for performance bonds	0	0
Workers Compensation Policy [Member]
Restricted Cash And Cash Equivalents Items [Line Items]
Collateral	$ 2,300,000	$ 2,200,000