Benefit Plans - Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Amounts within accumulated other comprehensive loss that had not yet been recognized as components of net periodic benefit cost
Tax impact	$ (48,720)	$ (63,040)	$ (57,461)	$ (33,463)
Pension Benefits
Amounts within accumulated other comprehensive loss that had not yet been recognized as components of net periodic benefit cost
Net actuarial loss	41,822	43,547	51,034
Total, pretax effect	41,822	43,547
Tax impact	(11,260)	(17,201)
Ending balance in accumulated other comprehensive loss	30,562	26,346
Amounts within accumulated other comprehensive loss expected to be recognized as components of net periodic benefit cost in next year
Amortization of net actuarial loss (gain)	6,320
Total to be recognized in 2019	6,320
Other Benefits
Amounts within accumulated other comprehensive loss that had not yet been recognized as components of net periodic benefit cost
Net actuarial loss	(2,507)	248	$ 282
Prior service credit	(480)	(909)
Total, pretax effect	(2,987)	(661)
Tax impact	804	261
Ending balance in accumulated other comprehensive loss	(2,183)	$ (400)
Amounts within accumulated other comprehensive loss expected to be recognized as components of net periodic benefit cost in next year
Amortization of prior service credit	(429)
Amortization of net actuarial loss (gain)	(304)
Total to be recognized in 2019	$ (733)