Defined Benefit Postretirement Plans (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Defined Benefit Postretirement Plans [Abstract]
Employer contributions	$ 0
Pension Benefits [Member]
Components of net periodic cost (benefit) [Abstract]
Service cost	402	$ 560
Interest cost	1,042	1,051
Expected return on plan assets	(1,985)	(1,835)
Net amortization	415	483
Total cost (benefit)	(126)	259
Other Benefits [Member]
Components of net periodic cost (benefit) [Abstract]
Service cost	3	4
Interest cost	86	94
Expected return on plan assets	0	0
Net amortization	20	29
Total cost (benefit)	$ 109	$ 127