Retirement Benefits - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Other Benefits
Components of net periodic benefit cost:
Service cost	$ 69	$ 72
Interest cost	444	500
Parent Company [Member] | Other Benefits
Components of net periodic benefit cost:
Service cost	69	72	$ 93
Interest cost	444	500	533
Expected return on plan assets	(472)	(461)	(446)
Amortization of prior service cost (credit)	0	0	0
Amortization of net loss (gain)	(335)	(402)	(386)
Net periodic benefit cost	(294)	(291)	(206)
Subsidiaries [Member] | Other Benefits
Components of net periodic benefit cost:
Service cost	69	72	93
Interest cost	444	500	533
Expected return on plan assets	(472)	(461)	(446)
Amortization of prior service cost (credit)	0	0	0
Amortization of net loss (gain)	(556)	(641)	(632)
Net periodic benefit cost	(515)	(530)	(452)
Nonqualified Plan | Qualified Pension Benefits
Components of net periodic benefit cost:
Service cost	847	913
Interest cost	2,120	2,285
Nonqualified Plan | Parent Company [Member] | Qualified Pension Benefits
Components of net periodic benefit cost:
Service cost	847	913	1,085
Interest cost	2,120	2,285	2,325
Expected return on plan assets	0	0	0
Amortization of prior service cost (credit)	1,580	42	42
Amortization of net loss (gain)	42	1,077	911
Net periodic benefit cost	4,589	4,317	4,363
Nonqualified Plan | Subsidiaries [Member] | Qualified Pension Benefits
Components of net periodic benefit cost:
Service cost	847	913	1,085
Interest cost	2,120	2,285	2,325
Expected return on plan assets	0	0	0
Amortization of prior service cost (credit)	44	44	44
Amortization of net loss (gain)	2,069	1,565	1,330
Net periodic benefit cost	5,080	4,807	4,784
Qualified Plan | Qualified Pension Benefits
Components of net periodic benefit cost:
Service cost	22,757	20,081
Interest cost	27,303	28,373
Qualified Plan | Parent Company [Member] | Qualified Pension Benefits
Components of net periodic benefit cost:
Service cost	22,757	20,081	18,913
Interest cost	27,303	28,373	28,689
Expected return on plan assets	(50,202)	(47,784)	(46,619)
Amortization of prior service cost (credit)	(1,980)	(1,980)	(1,980)
Amortization of net loss (gain)	2,187	0	0
Net periodic benefit cost	65	(1,310)	(997)
Qualified Plan | Subsidiaries [Member] | Qualified Pension Benefits
Components of net periodic benefit cost:
Service cost	22,757	20,081	18,913
Interest cost	27,303	28,373	28,689
Expected return on plan assets	(50,240)	(47,862)	(46,814)
Amortization of prior service cost (credit)	(1,573)	(1,573)	(1,573)
Amortization of net loss (gain)	14,917	13,048	15,257
Net periodic benefit cost	$ 13,164	$ 12,067	$ 14,472