Retirement Benefits - Textuals (Details) - Forecast [Member]	12 Months Ended
	Dec. 31, 2020 USD ($)
Other Pension Plan [Member]
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Amounts that Will be Amortized from Accumulated Other Comprehensive Income (Loss) in Next Fiscal Year, Net Gain (Loss)	$ (8,400,000)
Subsidiaries [Member] | Other Pension Plan [Member]
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Amounts that Will be Amortized from Accumulated Other Comprehensive Income (Loss) in Next Fiscal Year, Net Gain (Loss)	(200,000)
Qualified Plan | Subsidiaries [Member] | Pension Plan [Member]
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Amounts that Will be Amortized from Accumulated Other Comprehensive Income (Loss) in Next Fiscal Year, Net Gain (Loss)	(18,600,000)
Pension and Other Postretirement Benefit Plans, Net Prior Service Cost or Credit, Amounts that Will be Amortized from Accumulated Other Comprehensive Income (Loss) in Next Fiscal Year	1,600,000
Estimated Future Employer Contributions in Current Fiscal Year	18,000,000.0
Nonqualified Plan | Subsidiaries [Member] | Pension Plan [Member]
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Amounts that Will be Amortized from Accumulated Other Comprehensive Income (Loss) in Next Fiscal Year, Net Gain (Loss)	(2,600,000)
Pension and Other Postretirement Benefit Plans, Prior Service Cost, Amounts that Will be Amortized from Accumulated Other Comprehensive Income (Loss) in Next Fiscal Year	(300,000)
Estimated Future Employer Contributions in Current Fiscal Year	22,600,000
Other Pension Plan [Member] | Subsidiaries [Member] | Pension Plan [Member]
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Estimated Future Employer Contributions in Current Fiscal Year	$ 100,000