Pension and Other Postretirement Plans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Pension Benefits [Member]
Components of net periodic cost (benefit):
Service cost	$ 1,413	$ 1,878	$ 4,358	$ 5,715
Interest cost	3,161	3,594	9,744	10,936
Expected return on assets	(5,724)	(5,930)	(17,646)	(18,045)
Amortization of:
Prior service cost (benefit)	3	6	8	18
Actuarial loss	1,388	1,393	4,278	4,240
Net periodic cost (benefit)	241	941	742	2,864
Postretirement Benefits [Member]
Components of net periodic cost (benefit):
Service cost	361	194	1,084	580
Interest cost	753	392	2,258	1,176
Expected return on assets	(787)	(355)	(2,360)	(1,063)
Amortization of:
Transition obligation	1	0	3	1
Prior service cost (benefit)	(744)	(337)	(2,233)	(1,009)
Actuarial loss	178	133	533	397
Net periodic cost (benefit)	$ (238)	$ 27	$ (715)	$ 82