Accumulated Other Comprehensive Income (Loss) - Reclassifications Out 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 Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Tax (benefit) expense	$ 4,019	$ 19,761	$ 28,708	$ 54,220
Total reclassifications for the period, net of tax	(53,672)	(29,554)	(119,417)	(80,678)
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Non-operating expenses	6,427	5,133	19,281	15,398
Tax (benefit) expense	(1,654)	(1,997)	(4,964)	(5,990)
Total reclassifications for the period, net of tax	4,773	3,136	14,317	9,408
Actuarial Losses Recognized | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Non-operating expenses	6,837	5,573	20,510	16,718
Tax (benefit) expense	(1,760)	(2,168)	(5,279)	(6,503)
Total reclassifications for the period, net of tax	5,077	3,405	15,231	10,215
Prior Service Costs Recognized | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Non-operating expenses	(410)	(440)	(1,229)	(1,320)
Tax (benefit) expense	106	171	315	513
Total reclassifications for the period, net of tax	$ (304)	$ (269)	$ (914)	$ (807)