Employee Benefit Plans Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension Benefits [Member]
Components of Net Periodic Benefit Cost (Income) [Abstract]
Service cost	$ 3,204	$ 3,130
Interest cost	6,108	6,429
Expected return on plan assets	(7,060)	(6,008)
Amortization of prior service cost	1	2
Recognized actuarial loss	1,072	1,975
Net Periodic Benefit Cost (Income)	3,325	5,528
Other Postretirement Benefits [Member]
Components of Net Periodic Benefit Cost (Income) [Abstract]
Service cost	112	128
Interest cost	147	180
Expected return on plan assets	(239)	(213)
Amortization of prior service cost	(471)	(471)
Recognized actuarial loss	(20)	78
Net Periodic Benefit Cost (Income)	$ (471)	(298)
Nonservice cost component of net periodic benefit costs [Member]
Components of Net Periodic Benefit Cost (Income) [Abstract]
Net Periodic Benefit Cost (Income)		$ 2,600