Retirement Benefits Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Parent Company [Member] | Qualified Pension Benefits [Member]
Components of net periodic benefit cost:
Service cost	$ 6,172	$ 5,287	$ 12,169	$ 10,574	$ 22,656
Interest cost	6,292	7,216	12,590	14,433	28,913
Expected return on plan assets	(12,449)	(12,624)	(24,951)	(25,248)
Amortization of prior service cost	(292)	(495)	(787)	(990)
Amortization of net loss (gain)	2,099	251	4,080	501
Net periodic benefit cost	1,822	(365)	3,101	(730)
Parent Company [Member] | Supplemental Employee Retirement Plan [Member]
Components of net periodic benefit cost:
Service cost	176	256	404	512	1,023
Interest cost	362	578	740	1,157	2,314
Expected return on plan assets	0	0	0	0
Amortization of prior service cost	87	83	174	166
Amortization of net loss (gain)	512	341	1,098	683
Net periodic benefit cost	1,137	1,258	2,416	2,518
Parent Company [Member] | Other Benefit [Member]
Components of net periodic benefit cost:
Service cost	46	16	95	33	61
Interest cost	93	112	184	224	$ 410
Expected return on plan assets	(98)	(98)	(195)	(195)
Amortization of prior service cost	0	0	0	0
Amortization of net loss (gain)	(19)	(63)	(41)	(125)
Net periodic benefit cost	22	(33)	43	(63)
Subsidiaries [Member] | Qualified Pension Benefits [Member]
Components of net periodic benefit cost:
Service cost	6,172	5,287	12,169	10,574
Interest cost	6,292	7,216	12,590	14,433
Expected return on plan assets	(12,451)	(12,628)	(24,955)	(25,257)
Amortization of prior service cost	(393)	(393)	(787)	(787)
Amortization of net loss (gain)	4,866	3,165	9,522	6,330
Net periodic benefit cost	4,486	2,647	8,539	5,293
Subsidiaries [Member] | Supplemental Employee Retirement Plan [Member]
Components of net periodic benefit cost:
Service cost	176	256	404	512
Interest cost	362	578	740	1,157
Expected return on plan assets	0	0	0	0
Amortization of prior service cost	87	83	174	167
Amortization of net loss (gain)	575	433	1,234	866
Net periodic benefit cost	1,200	1,350	2,552	2,702
Subsidiaries [Member] | Other Benefit [Member]
Components of net periodic benefit cost:
Service cost	46	16	95	33
Interest cost	93	112	184	224
Expected return on plan assets	(98)	(98)	(195)	(195)
Amortization of prior service cost	0	0	0	0
Amortization of net loss (gain)	(33)	(109)	(69)	(219)
Net periodic benefit cost	$ 8	$ (79)	$ 15	$ (157)