Accumulated Other Comprehensive Income (Loss) - Reclassifications of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense	$ (19,815)	$ (16,133)	$ (55,995)	$ (35,574)
Amounts Reclassified | Prior service cost
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Other income	113	113	226	227
Amounts Reclassified | Actuarial losses
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Other income	(31)	(2)	(62)	(5)
Amounts Reclassified | Accumulated Defined Benefit Plans Adjustment Including Portion Attributable to Noncontrolling Interest
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Total postretirement plans	82	111	164	222
Income tax expense	(20)	(42)	(41)	(84)
Total reclassifications for the period, net of tax	$ 62	$ 69	$ 123	$ 138