Pension and Other Postretirement Plans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Pension Benefits [Member]
Components of net periodic cost (benefit):
Service cost	$ 2,039	$ 1,589	$ 3,837	$ 3,073
Interest cost	3,902	3,483	7,342	6,735
Expected return on assets	(6,438)	(5,656)	(12,115)	(10,935)
Amortization of:
Prior service (benefit) cost	6	53	12	102
Actuarial loss	1,513	186	2,847	358
Net periodic cost (benefit)	1,022	(345)	1,923	(667)
Postretirement Benefits [Member]
Components of net periodic cost (benefit):
Service cost	3	152	386	482
Interest cost	6	356	784	1,127
Expected return on assets	(5)	(301)	(708)	(955)
Amortization of:
Transition obligation	0	0	1	1
Prior service (benefit) cost	(5)	(307)	(672)	(974)
Actuarial loss	2	17	264	57
Net periodic cost (benefit)	$ 1	$ (83)	$ 55	$ (262)