Benefit Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension Benefits
Components of net periodic benefit cost
Service cost	$ 157	$ 214	$ 315	$ 429
Interest cost	2,042	2,157	4,083	4,314
Expected return on plan assets	(1,242)	(1,164)	(2,484)	(2,335)
Recognized net actuarial loss	1,999	1,774	3,998	3,545
Total net periodic benefit cost	2,956	2,981	5,912	5,953
Other Benefits
Components of net periodic benefit cost
Service cost	179	192	358	384
Interest cost	201	211	402	421
Prior service credit	(108)	(107)	(215)	(214)
Total net periodic benefit cost	$ 272	$ 296	$ 545	$ 591