Benefit Plans - Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Amounts within accumulated other comprehensive loss that had not yet been recognized as components of net periodic benefit cost
Tax impact	$ (63,040)	$ (57,461)	$ (33,463)	$ (33,591)
Pension Benefits
Amounts within accumulated other comprehensive loss that had not yet been recognized as components of net periodic benefit cost
Net actuarial loss	43,547	51,034	45,579
Total, pretax effect	43,547	51,034
Tax impact	(17,201)	(20,158)
Ending balance in accumulated other comprehensive loss	26,346	30,876
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	6,440
Total to be recognized in 2018	6,440
Other Benefits
Amounts within accumulated other comprehensive loss that had not yet been recognized as components of net periodic benefit cost
Net actuarial loss	248	282	$ 619
Prior service credit	(909)	(1,338)
Total, pretax effect	(661)	(1,056)
Tax impact	261	417
Ending balance in accumulated other comprehensive loss	(400)	$ (639)
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)
Total to be recognized in 2018	$ (429)