Retirement Benefits Change in Net Benefit Obligation (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Qualified Pension Benefits [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Defined Benefit Plan, Benefit Obligation, Beginning Balance	$ 849,383	$ 774,305
Defined Benefit Plan, Service Cost	6,711	24,337
Defined Benefit Plan, Interest Cost	5,578	25,180
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)	0	69,413
Defined Benefit Plan, Benefit Obligation, Benefits Paid	(11,625)	(42,775)
Defined Benefit Plan, Benefit Obligation, Prescription Drug Subsidy Receipt	0	0
defined benefit plan expected administration expense	0	1,077
Defined Benefit Plan, Benefit Obligation, Ending Balance	850,047	849,383
Supplemental Employee Retirement Plan [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Defined Benefit Plan, Benefit Obligation, Beginning Balance	46,742	63,000
Defined Benefit Plan, Service Cost	115	756
Defined Benefit Plan, Interest Cost	293	1,464
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)	0	3,663
Defined Benefit Plan, Benefit Obligation, Benefits Paid	(496)	(22,141)
Defined Benefit Plan, Benefit Obligation, Prescription Drug Subsidy Receipt	0	0
defined benefit plan expected administration expense	0	0
Defined Benefit Plan, Benefit Obligation, Ending Balance	46,654	46,742
Other Benefit [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Defined Benefit Plan, Benefit Obligation, Beginning Balance	12,114	11,627
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment	0	(44)
Defined Benefit Plan, Service Cost	41	190
Defined Benefit Plan, Interest Cost	77	368
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss)	0	604
Defined Benefit Plan, Benefit Obligation, Benefits Paid	(237)	(906)
Defined Benefit Plan, Benefit Obligation, Prescription Drug Subsidy Receipt	196	187
defined benefit plan expected administration expense	0	0
Defined Benefit Plan, Benefit Obligation, Ending Balance	12,191	12,114
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Curtailment	0	0
Qualified Plan [Member] | Qualified Pension Benefits [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment	0	0
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Curtailment	0	0
Nonqualified Plan [Member] | Qualified Pension Benefits [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Curtailment	0	0
Nonqualified Plan [Member] | Supplemental Employee Retirement Plan [Member]
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment	$ 0	$ 0