Retirement Benefits - Change in Net Benefit Obligation and Fair Value (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment	$ 0	$ 0
Change in benefit obligation:
Benefit obligation at beginning of period	11,454		$ 11,194
Service cost	69	72
Interest cost	444	500
Actuarial loss (gain)	(379)	725
Benefits paid	(1,037)	(1,137)
Medicare part D subsidy received	85	100
Defined Benefit Plan, Expected Administration Expenses	0	0
Benefit obligation at end of period	10,636	11,454
Change in plan assets:
Fair value of plan assets at beginning of period	7,138		7,200
Actual return on plan assets	(395)	784
Employer contribution	254	291
Benefits paid	(1,037)	(1,137)
Defined Benefit Plan, Plan Assets, Administration Expense	0	0
Fair value of plan assets at end of period	5,960	7,138
Funded status at end of period	(4,676)	(4,316)
Subsidiaries [Member] | Other Benefits
Change in benefit obligation:
Service cost	69	72	93
Interest cost	444	500	533
Nonqualified Plan | Qualified Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment	(1,446)	0
Change in benefit obligation:
Benefit obligation at beginning of period	55,754		51,734
Service cost	847	913
Interest cost	2,120	2,285
Actuarial loss (gain)	1,122	2,722
Benefits paid	(5,581)	(1,900)
Medicare part D subsidy received	0	0
Defined Benefit Plan, Expected Administration Expenses	0	0
Benefit obligation at end of period	55,708	55,754
Change in plan assets:
Fair value of plan assets at beginning of period	0		0
Actual return on plan assets	0	0
Employer contribution	5,581	1,900
Benefits paid	(5,581)	(1,900)
Defined Benefit Plan, Plan Assets, Administration Expense	0	0
Fair value of plan assets at end of period	0	0
Funded status at end of period	(55,708)	(55,754)
Nonqualified Plan | Subsidiaries [Member] | Qualified Pension Benefits
Change in benefit obligation:
Service cost	847	913	1,085
Interest cost	2,120	2,285	2,325
Qualified Plan | Qualified Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment	0	0
Change in benefit obligation:
Benefit obligation at beginning of period	700,481		652,607
Service cost	22,757	20,081
Interest cost	27,303	28,373
Actuarial loss (gain)	(29,067)	40,945
Benefits paid	(42,662)	(40,594)
Medicare part D subsidy received	0	0
Defined Benefit Plan, Expected Administration Expenses	(1,169)	(931)
Benefit obligation at end of period	677,643	700,481
Change in plan assets:
Fair value of plan assets at beginning of period	704,360		620,260
Actual return on plan assets	(38,379)	107,836
Employer contribution	18,000	18,000
Benefits paid	(42,662)	(40,594)
Defined Benefit Plan, Plan Assets, Administration Expense	(1,077)	(1,142)
Fair value of plan assets at end of period	640,242	704,360
Funded status at end of period	(37,401)	3,879
Qualified Plan | Subsidiaries [Member] | Qualified Pension Benefits
Change in benefit obligation:
Service cost	22,757	20,081	18,913
Interest cost	$ 27,303	$ 28,373	$ 28,689