Benefit Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension Benefits
Components of net periodic benefit cost
Service cost	$ 157	$ 215	$ 472	$ 644
Interest cost	2,041	2,172	6,124	6,486
Expected return on plan assets	(1,242)	(1,180)	(3,726)	(3,515)
Recognized net actuarial loss	2,000	1,774	5,998	5,319
Total net periodic benefit cost	2,956	2,981	8,868	8,934
Other Benefits
Components of net periodic benefit cost
Service cost	179	193	537	577
Interest cost	201	210	603	631
Prior service credit	(107)	(107)	(322)	(321)
Total net periodic benefit cost	$ 273	$ 296	$ 818	$ 887