Employee Retirement Plans - Changes in Healthcare Cost Trend Rate (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($)
Pension plan
Amounts included in other comprehensive loss, net of tax
Net actuarial gain (loss)	$ (6,414)
Amounts that will be amortized from accumulated other comprehensive income loss into net periodic pension or OPEB cost in the next fiscal year
Estimated actuarial gain (loss)	(723)
Other postretirement benefit cost
Effect of one percentage point change in the healthcare cost trend rate
Effect on total service and interest cost, 1% increase	62
Effect on total service and interest cost, 1% decrease	(54)
Effect on postretirement benefit obligation, 1% increase	822
Effect on postretirement benefit obligation, 1% decrease	(721)
Amounts included in other comprehensive loss, net of tax
Net actuarial gain (loss)	937
Amounts that will be amortized from accumulated other comprehensive income loss into net periodic pension or OPEB cost in the next fiscal year
Estimated actuarial gain (loss)	$ 107