Pension and Other Retirement Plans - Amounts Included in Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Unrecognized net actuarial (losses) gains, net of tax	$ (46,721)	$ (66,397)
Unrecognized prior service (cost) credit, net of tax	909	1,514
Total	(45,812)	(64,883)
Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Unrecognized net actuarial (losses) gains, net of tax	(46,897)	(66,212)
Unrecognized prior service (cost) credit, net of tax	(942)	(568)
Total	(47,839)	(66,780)
Other Postretirement Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Unrecognized net actuarial (losses) gains, net of tax	176	(185)
Unrecognized prior service (cost) credit, net of tax	1,851	2,082
Total	$ 2,027	$ 1,897