Retirement Benefits Change in Net Benefit Obligation (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Qualified Pension Benefits [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of period	$ 700,481	$ 652,607
Service cost	17,068	20,081
Interest cost	20,477	28,373
Actuarial loss/(gain)	3,991	40,945
Benefits paid	(33,183)	(40,594)
Medicare part D subsidiary received	0	0
Administrative Expense	0	(931)
Benefit obligation at end of period	708,834	700,481
Supplemental Employee Retirement Plan [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of period	55,754	51,734
Service cost	635	913
Interest cost	1,590	2,285
Actuarial loss/(gain)	0	2,722
Benefits paid	(1,561)	(1,900)
Medicare part D subsidiary received	0	0
Administrative Expense	0	0
Benefit obligation at end of period	56,418	55,754
Other Benefit [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of period	11,454	11,194
Service cost	52	72
Interest cost	333	500
Actuarial loss/(gain)	82	725
Benefits paid	(816)	(1,137)
Medicare part D subsidiary received	85	100
Administrative Expense	0	0
Benefit obligation at end of period	$ 11,190	$ 11,454