Retirement Benefits - Benefit Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Subsidiaries [Member]
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer matching contribution, percent	4.50%
Defined Contribution Plan, Interest Credit	1.00%
Subsidiaries [Member] | employer contribution [Member]
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer matching contribution, percent	4.00%
Subsidiaries [Member] | employer contribution [Member] | Collective Bargaining Arrangement Member [Member]
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer matching contribution, percent	4.00%
Subsidiaries [Member] | employer contribution [Member] | Collective Bargaining Arrangement Member [Member] | UA represented [Member]
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer matching contribution, percent	4.00%
Subsidiaries [Member] | employer contribution [Member] | Collective Bargaining Arrangement Member [Member] | IBEW represented [Member]
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer matching contribution, percent	4.00%
Other Pension Plan [Member]
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net loss (gain)	$ 900	$ (488)
Amortization of net (loss) gain	374	335
Other Comprehensive Income (Loss), Defined Benefit Plan, Settlement and Curtailment Gain (Loss), before Tax	2,892	0
Prior service cost (credit)	0	0
Amortization of prior service (cost) credit	0	0
Total change in other comprehensive income for year	2,366	823
Other Pension Plan [Member] | Subsidiaries [Member]
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net loss (gain)	900	(488)
Amortization of net (loss) gain	562	556
Other Comprehensive Income (Loss), Defined Benefit Plan, Settlement and Curtailment Gain (Loss), before Tax	3,832	0
Prior service cost (credit)	0	0
Amortization of prior service (cost) credit	0	0
Total change in other comprehensive income for year	3,494	1,044
Qualified Plan | Pension Plan [Member]
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net loss (gain)	(541)	(59,422)
Amortization of net (loss) gain	(1,151)	(2,187)
Other Comprehensive Income (Loss), Defined Benefit Plan, Settlement and Curtailment Gain (Loss), before Tax	0	0
Prior service cost (credit)	0	0
Amortization of prior service (cost) credit	(1,980)	(1,980)
Total change in other comprehensive income for year	1,370	59,215
Qualified Plan | Pension Plan [Member] | Subsidiaries [Member]
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net loss (gain)	(559)	(59,460)
Amortization of net (loss) gain	(12,877)	(14,917)
Other Comprehensive Income (Loss), Defined Benefit Plan, Settlement and Curtailment Gain (Loss), before Tax	0	0
Prior service cost (credit)	0	0
Amortization of prior service (cost) credit	(1,573)	(1,573)
Total change in other comprehensive income for year	(10,745)	46,116
Nonqualified Plan | Pension Plan [Member]
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net loss (gain)	(6,756)	(1,122)
Amortization of net (loss) gain	(1,365)	(1,580)
Other Comprehensive Income (Loss), Defined Benefit Plan, Settlement and Curtailment Gain (Loss), before Tax	0	(619)
Prior service cost (credit)	0	1,446
Amortization of prior service (cost) credit	331	42
Total change in other comprehensive income for year	5,060	327
Nonqualified Plan | Pension Plan [Member] | Subsidiaries [Member]
Defined Benefit Plan and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net loss (gain)	(6,756)	(1,122)
Amortization of net (loss) gain	(1,733)	(2,069)
Other Comprehensive Income (Loss), Defined Benefit Plan, Settlement and Curtailment Gain (Loss), before Tax	0	(737)
Prior service cost (credit)	0	1,446
Amortization of prior service (cost) credit	333	44
Total change in other comprehensive income for year	$ 4,690	$ (282)