Retirement Benefits Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Qualified Pension Benefits [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Defined Benefit Plan, Service Cost	$ 6,711		$ 24,337
Defined Benefit Plan, Interest Cost	5,578		25,180
Qualified Pension Benefits [Member] | Parent Company [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Defined Benefit Plan, Service Cost	6,711	$ 5,997
Defined Benefit Plan, Interest Cost	5,578	6,298
Defined Benefit Plan, Expected Return (Loss) on Plan Assets	(12,081)	(12,502)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	(476)	(495)
Defined Benefit Plan, Amortization of Gain (Loss)	2,830	1,981
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Total	2,562	1,279
Qualified Pension Benefits [Member] | Subsidiaries [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Defined Benefit Plan, Service Cost	6,711	5,997
Defined Benefit Plan, Interest Cost	5,578	6,298
Defined Benefit Plan, Expected Return (Loss) on Plan Assets	(12,081)	(12,504)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	(378)	(393)
Defined Benefit Plan, Amortization of Gain (Loss)	5,311	4,656
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Total	5,141	4,054
Supplemental Employee Retirement Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Defined Benefit Plan, Service Cost	115		756
Defined Benefit Plan, Interest Cost	293		1,464
Supplemental Employee Retirement Plan [Member] | Parent Company [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Defined Benefit Plan, Service Cost	115	228
Defined Benefit Plan, Interest Cost	293	378
Defined Benefit Plan, Expected Return (Loss) on Plan Assets	0	0
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	87	87
Defined Benefit Plan, Amortization of Gain (Loss)	587	586
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Total	1,082	1,279
Supplemental Employee Retirement Plan [Member] | Subsidiaries [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Defined Benefit Plan, Service Cost	115	228
Defined Benefit Plan, Interest Cost	293	378
Defined Benefit Plan, Expected Return (Loss) on Plan Assets	0	0
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	87	87
Defined Benefit Plan, Amortization of Gain (Loss)	635	659
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Total	1,130	1,352
Other Benefit [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Defined Benefit Plan, Service Cost	41		190
Defined Benefit Plan, Interest Cost	77		$ 368
Other Benefit [Member] | Parent Company [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Defined Benefit Plan, Service Cost	41	49
Defined Benefit Plan, Interest Cost	77	91
Defined Benefit Plan, Expected Return (Loss) on Plan Assets	(91)	(97)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	2	0
Defined Benefit Plan, Amortization of Gain (Loss)	(10)	(22)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Total	19	21
Other Benefit [Member] | Subsidiaries [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Defined Benefit Plan, Service Cost	41	49
Defined Benefit Plan, Interest Cost	77	91
Defined Benefit Plan, Expected Return (Loss) on Plan Assets	(91)	(97)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	2	0
Defined Benefit Plan, Amortization of Gain (Loss)	(15)	(36)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Total	$ 14	$ 7