Comprehensive Income and Accumulated Other Comprehensive Income (Loss) - Summary of Effects of Reclassifications from Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Amortization of pension and other postretirement benefit plan items:
Total net of tax	$ 1,567	$ 6,336
Settlement gain [Member]
Amortization of pension and other postretirement benefit plan items:
Total before tax	(8)
Actuarial losses [Member]
Amortization of pension and other postretirement benefit plan items:
Total before tax	2,597
Prior service credit [Member]
Amortization of pension and other postretirement benefit plan items:
Total before tax	(41)
Pension and Other Postretirement Benefit Plans [Member]
Amortization of pension and other postretirement benefit plan items:
Total before tax	2,548
Tax benefit	(981)
Total net of tax	$ 1,567	$ 6,336