Employee Benefit Plans - Components of Net Periodic Benefit Cost (Details) - Postretirement Benefit Plans - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2017	Jun. 30, 2016
Components of net periodic benefit cost:
Service cost	$ 18	$ 24		$ 35	$ 47
Interest cost	57	52		113	118
Expected long-term return on assets	(62)	(69)		(124)	(180)
Prior service cost amortization	(114)	(113)		(227)	(238)
Actuarial loss	2	0		5	0
Settlement charge	0	0		0	45
Curtailment gain	0	0	$ (800)	0	(816)
Net periodic (income) expense	$ (99)	$ (106)		$ (198)	$ (1,024)