Commitments And Contingencies (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 30, 2014
Loss Contingencies [Line Items]
Commitment for property and equipment purchases	$ 236,800		$ 236,800
Loss contingency, damages awarded, value					$ 800
Insurance and claims	30,689	$ 19,827	51,847	$ 39,667
Loss contingency, damages awarded, attorney fees and costs					$ 500
May 17, 2018 Verdict [Member]
Loss Contingencies [Line Items]
Loss contingency, damages awarded, value	89,700
Self insurance retained liability						$ 10,000
Insurance and claims	11,300
Litigation pre-judgment interest	1,300
Loss contingency, receivable, noncurrent	79,700		79,700
Loss contingency, accrual, noncurrent	$ 79,700		$ 79,700