Employee Retirement Plans - Assumptions (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Weighted average assumptions used to determine benefit obligations as of December 31:
Compensation increase rate	3.54%	3.55%
Weighted average assumptions used to determine net cost for years ended December 31:
Compensation increase rate	3.55%	3.55%
Pension
Weighted average assumptions used to determine benefit obligations as of December 31:
Discount rate	3.55%	4.02%
Weighted average assumptions used to determine net cost for years ended December 31:
Discount rate	4.02%	4.43%
Expected return on plan assets (as a percent)	6.54%	6.66%
Postretirement
Weighted average assumptions used to determine benefit obligations as of December 31:
Discount rate	3.42%	3.90%
Weighted average assumptions used to determine net cost for years ended December 31:
Discount rate	3.90%	4.24%