Accumulated Other Comprehensive Income (Loss) - Reclassifications Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Tax (benefit) expense	$ 8,115	$ 11,996
Total reclassifications for the period, net of tax	(21,164)	(17,723)
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Non-operating expenses	6,525	5,075
Tax (benefit) expense	(1,680)	(1,974)
Total reclassifications for the period, net of tax	4,845	3,101
Actuarial Losses Recognized | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Non-operating expenses	6,950	5,525
Tax (benefit) expense	(1,789)	(2,149)
Total reclassifications for the period, net of tax	5,161	3,376
Prior Service Costs Recognized | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Non-operating expenses	(425)	(450)
Tax (benefit) expense	109	175
Total reclassifications for the period, net of tax	$ (316)	$ (275)