Benefit Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension Benefits
Components of net periodic benefit cost
Service cost	$ 944	$ 809	$ 702
Interest cost	8,784	8,681	8,995
Expected return on plan assets	(4,698)	(4,178)	(4,270)
Recognized net actuarial loss	7,629	9,928	5,163
Total net periodic benefit cost	12,659	15,240	10,590
Other Benefits
Components of net periodic benefit cost
Service cost	697	734	742
Interest cost	812	770	925
Prior service credit	(429)	(429)
Recognized net actuarial loss		32
Total net periodic benefit cost	1,080	1,107	1,667
Pension Plan
Components of net periodic benefit cost
Interest cost	4,182	4,252	4,461
Expected return on plan assets	(4,698)	(4,178)	(4,270)
Recognized net actuarial loss	3,443	4,225	1,826
Total net periodic benefit cost	$ 2,927	$ 4,299	$ 2,017