Pension and Other Postretirement Plans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Pension Benefits [Member]
Components of net periodic cost (benefit):
Service cost	$ 1,564	$ 2,039	$ 2,945	$ 3,837
Interest cost	3,494	3,902	6,583	7,342
Expected return on assets	(6,329)	(6,438)	(11,922)	(12,115)
Amortization of:
Prior service cost (benefit)	3	6	5	12
Actuarial loss	1,534	1,513	2,890	2,847
Net periodic cost (benefit)	266	1,022	501	1,923
Postretirement Benefits [Member]
Components of net periodic cost (benefit):
Service cost	399	3	723	386
Interest cost	831	6	1,505	784
Expected return on assets	(868)	(5)	(1,573)	(708)
Amortization of:
Transition obligation	1	0	2	1
Prior service cost (benefit)	(822)	(5)	(1,489)	(672)
Actuarial loss	196	2	355	264
Net periodic cost (benefit)	$ (263)	$ 1	$ (477)	$ 55