Retirement Benefits Change in Net Benefit Obligation (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 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	10,851	20,081
Interest cost	13,560	28,373
Actuarial loss/(gain)	0	40,945
Benefits paid	(21,300)	(40,594)
Medicare part D subsidiary received	0	0
Administrative Expense	0	(931)
Benefit obligation at end of period	703,592	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	423	913
Interest cost	1,060	2,285
Actuarial loss/(gain)	0	2,722
Benefits paid	(1,080)	(1,900)
Medicare part D subsidiary received	0	0
Administrative Expense	0	0
Benefit obligation at end of period	56,157	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	35	72
Interest cost	220	500
Actuarial loss/(gain)	0	725
Benefits paid	(562)	(1,137)
Medicare part D subsidiary received	85	100
Administrative Expense	0	0
Benefit obligation at end of period	$ 11,232	$ 11,454