Employee Retirement Plans - Assumptions (Details)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension plan
Assumptions used in determining net periodic pension cost for the plans
Discount rates (as a percent)	4.50%
Expected long-term rate of return on assets (as a percent)	7.25%	7.25%	7.25%
Expected long-term rate of return on plan assets for the next fiscal year	6.50
Pension plan | Minimum
Assumptions used in determining net periodic pension cost for the plans
Discount rates (as a percent)		3.90%	4.80%
Pension plan | Maximum
Assumptions used in determining net periodic pension cost for the plans
Discount rates (as a percent)		4.00%	4.90%
Salaried Pension Plan
Assumptions used in determining net periodic pension cost for the plans
Rates of increase in compensation levels: Salaried (as a percent)	3.50%	3.50%	3.50%
Assumptions used to determine the benefit obligation
Discount rate (as a percent)		4.50%
Hourly Pension Plan
Assumptions used to determine the benefit obligation
Discount rate (as a percent)	4.20%	4.50%
Other postretirement benefit cost
Assumptions used in determining net periodic pension cost for the plans
Discount rates (as a percent)	4.10%	3.70%	4.50%
Summary of healthcare cost trend rates
Ultimate healthcare cost trend rate (as a percent)	4.50%	4.50%	4.50%
Participation (as a percent)	60.00%	60.00%	60.00%
Assumed health care cost trend rate for the next fiscal year (as a percent)	7.00%	7.00%
Ultimate rate in 2025 (as a percent)	4.50%
Ultimate rate in 2024 (as a percent)		4.50%
Ultimate rate in 2023 (as a percent)			4.50%
Assumptions used to determine the benefit obligation
Discount rate (as a percent)	3.70%	4.50%
Other postretirement benefit cost | Under 65
Summary of healthcare cost trend rates
Assumed health care cost trend rate for the next fiscal year (as a percent)			7.00%
Other postretirement benefit cost | Over 65
Summary of healthcare cost trend rates
Assumed health care cost trend rate for the next fiscal year (as a percent)			6.00%