Labor Union Contracts (Details)	12 Months Ended
	Dec. 31, 2017
Workforce subject to collective bargaining arrangements expiring within one year
Labor Union Contracts
Number of unionized employees	1,000
Workforce subject to collective bargaining agreements, expired in 2017
Labor Union Contracts
Number of unionized employees	1,000
Workforce subject to collective bargaining arrangements, effective June 18, 2017
Labor Union Contracts
Number of unionized employees	10,000
Total number of employees | Unionized employees concentration risk
Labor Union Contracts
Concentration risk (percent)	28.00%
Total number of employees | Unionized employees concentration risk | Workforce subject to collective bargaining arrangements, effective June 18, 2017
Labor Union Contracts
Concentration risk (percent)	71.00%