Pension and Other Postretirement Plans (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Pension Benefits [Member]
Components of net periodic cost (benefit):
Service cost	$ 1,381	$ 1,798
Interest cost	3,089	3,440
Expected return on assets	(5,593)	(5,677)
Amortization of:
Prior service cost (benefit)	2	6
Actuarial loss	1,356	1,334
Net periodic cost (benefit)	235	901
Postretirement Benefits [Member]
Components of net periodic cost (benefit):
Service cost	324	383
Interest cost	674	778
Expected return on assets	(705)	(703)
Amortization of:
Transition obligation	1	1
Prior service cost (benefit)	(667)	(667)
Actuarial loss	159	262
Net periodic cost (benefit)	$ (214)	$ 54