Employee Retirement Plans - Assumptions (Details)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension
Weighted average assumptions used to determine benefit obligations as of December 31:
Discount rate	4.02%	4.43%	4.10%
Compensation increase rate	3.55%	3.55%
Weighted average assumptions used to determine net cost for years ended December 31:
Discount rate	4.43%	4.10%
Expected return on plan assets (as a percent)	6.66%	6.70%
Compensation increase rate	3.55%	3.50%
Postretirement
Weighted average assumptions used to determine benefit obligations as of December 31:
Discount rate	3.90%	4.24%
Weighted average assumptions used to determine net cost for years ended December 31:
Discount rate	4.24%	3.70%