Retirement Benefits - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Other Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	$ 61	$ 69
Interest cost	410	444
Parent Company [Member] | Other Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	61	69	$ 72
Interest cost	410	444	500
Expected return on plan assets	(393)	(472)	(461)
Amortization of prior service cost (credit)	0	0	0
Amortization of net loss (gain)	(374)	(335)	(402)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Total	(296)	(294)	(291)
Subsidiaries [Member] | Other Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	61	69	72
Interest cost	410	444	500
Expected return on plan assets	(393)	(472)	(461)
Amortization of prior service cost (credit)	0	0	0
Amortization of net loss (gain)	(562)	(556)	(641)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Total	(484)	(515)	(530)
Nonqualified Plan | Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	1,023	847
Interest cost	2,314	2,120
Nonqualified Plan | Parent Company [Member] | Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	1,023	847	913
Interest cost	2,314	2,120	2,285
Expected return on plan assets	0	0	0
Amortization of prior service cost (credit)	331	1,580	42
Amortization of net loss (gain)	1,365	42	1,077
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Total	5,033	4,589	4,317
Nonqualified Plan | Subsidiaries [Member] | Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	1,023	847	913
Interest cost	2,314	2,120	2,285
Expected return on plan assets	0	0	0
Amortization of prior service cost (credit)	333	44	44
Amortization of net loss (gain)	1,733	2,069	1,565
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Total	5,403	5,080	4,807
Qualified Plan | Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	22,656	22,757
Interest cost	28,913	27,303
Qualified Plan | Parent Company [Member] | Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	22,656	22,757	20,081
Interest cost	28,913	27,303	28,373
Expected return on plan assets	(50,249)	(50,202)	(47,784)
Amortization of prior service cost (credit)	(1,980)	(1,980)	(1,980)
Amortization of net loss (gain)	1,151	2,187	0
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Total	491	65	(1,310)
Qualified Plan | Subsidiaries [Member] | Pension Plan [Member]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	22,656	22,757	20,081
Interest cost	28,913	27,303	28,373
Expected return on plan assets	(50,267)	(50,240)	(47,862)
Amortization of prior service cost (credit)	(1,573)	(1,573)	(1,573)
Amortization of net loss (gain)	12,877	14,917	13,048
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Total	$ 12,606	$ 13,164	$ 12,067