Benefit Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Components of net periodic benefit cost
Service cost	$ 629	$ 944	$ 809
Interest cost	8,297	8,784	8,681
Expected return on plan assets	(5,003)	(4,698)	(4,178)
Recognized net actuarial loss	8,625	7,629	9,928
Total net periodic benefit cost	12,548	12,659	15,240
Pension Benefits | Funded Plan
Components of net periodic benefit cost
Interest cost	3,922	4,182	4,252
Expected return on plan assets	(5,003)	(4,698)	(4,178)
Recognized net actuarial loss	4,316	3,443	4,225
Total net periodic benefit cost	3,235	2,927	4,299
Other Benefits
Components of net periodic benefit cost
Service cost	738	697	734
Interest cost	776	812	770
Prior service credit	(429)	(429)	(429)
Recognized net actuarial loss			32
Total net periodic benefit cost	$ 1,085	$ 1,080	$ 1,107