Employee Benefit Plans (Consolidated Net Periodic Benefit Costs) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plan [Member]
Components of net periodic pension and postretirement benefit expense:
Service cost	$ 20,497	$ 18,179	$ 18,890
Interest cost	9,865	7,950	7,754
Expected return on plan assets	(10,266)	(8,913)	(8,037)
Amortization of prior service cost (credit)	(181)	(181)	0
Amortization of net actuarial loss	5,622	4,645	6,306
Settlement	2,460	202	0
Net periodic expense (credit)	27,997	21,882	24,913
Other Postretirement Benefits [Member]
Components of net periodic pension and postretirement benefit expense:
Service cost	243	235	243
Interest cost	475	489	438
Expected return on plan assets	0	0	0
Amortization of prior service cost (credit)	0	(3,335)	(3,713)
Amortization of net actuarial loss	749	880	885
Settlement	0	0	0
Net periodic expense (credit)	$ 1,467	$ (1,731)	$ (2,147)