Accumulated Other Comprehensive Income (Loss) - Reclassifications of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense	$ 5,300	$ 9,206	$ (50,695)	$ (26,368)
Amounts Reclassified | Prior service cost
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Other income	113	113	340	341
Amounts Reclassified | Actuarial losses
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Other income	(31)	(2)	(94)	(7)
Amounts Reclassified | Adjustments
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Total postretirement plans	82	111	246	334
Income tax expense	(20)	(42)	(62)	(126)
Total reclassifications for the period, net of tax	$ 62	$ 69	$ 184	$ 208