Supplemental Stockholders' Equity Information - Reclassifications Out of Accumulated Other Comprehensive Income (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($)
Amortization of prior service credit
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from accumulated other comprehensive loss, before tax	$ (6,229)
Amortization of actuarial loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from accumulated other comprehensive loss, before tax	27,555
Other
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from accumulated other comprehensive loss, before tax	421
Total reclassifications
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from accumulated other comprehensive loss, before tax	21,747
Income tax expense	5,692
Total reclassification, net of tax	$ 16,055