Retirement Benefits Change in Net Benefit Obligation (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2016
Qualified Pension Benefits [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of period		$ 652,607
Service cost	$ 5,425	20,081
Interest cost	6,780	28,373
Actuarial loss/(gain)	0	40,945
Benefits paid	(10,650)	(40,594)
Medicare part D subsidiary received	0	0
Administrative Expense	0	(931)
Benefit obligation at end of period	702,036	700,481
Supplemental Employee Retirement Plan [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of period		51,734
Service cost	212	913
Interest cost	530	2,285
Actuarial loss/(gain)	0	2,722
Benefits paid	(592)	(1,900)
Medicare part D subsidiary received	0	0
Administrative Expense	0	0
Benefit obligation at end of period	55,904	55,754
Other Benefit [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of period		11,194
Service cost	17	72
Interest cost	110	500
Actuarial loss/(gain)	0	725
Benefits paid	(279)	(1,137)
Medicare part D subsidiary received	0	100
Administrative Expense	0	0
Benefit obligation at end of period	$ 11,302	$ 11,454