Benefit Plans - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Pension Benefits
Components of net periodic benefit cost
Service cost	$ 158	$ 215
Interest cost	2,041	2,157
Expected return on plan assets	(1,242)	(1,171)
Recognized net actuarial loss	1,999	1,771
Total net periodic benefit cost	2,956	2,972
Other Benefits
Components of net periodic benefit cost
Service cost	179	192
Interest cost	201	210
Prior service credit	(107)	(107)
Total net periodic benefit cost	$ 273	$ 295