Restricted Cash (Narrative) (Details) - USD ($)	Jun. 30, 2017	Dec. 31, 2016
Restricted Cash [Abstract]
Surety, payment and performance bonds	$ 38,900,000	$ 39,200,000
Collateral payment for performance bonds	0	500,000
Collateral related to workers compensation policy	$ 2,500,000	$ 2,500,000