Pension, Savings, and Other Employee Benefits - Schedule of Assumptions Used in the Defined Benefit Plans (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Minimum required outstanding par value to each issue of bonds	$ 300,000,000
Qualified pension
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan assumptions used calculating benefit obligation, discount rate	2.63%	3.31%	4.43%
Defined benefit plan assumptions used calculating net periodic benefit cost, discount rate	3.31%	4.43%	3.75%
Nonqualified pension
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan assumptions used calculating benefit obligation, discount rate	2.24%	3.08%	4.26%
Defined benefit plan assumptions used calculating net periodic benefit cost, discount rate	3.08%	4.26%	3.59%
Other nonqualified pension
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan assumptions used calculating benefit obligation, discount rate	1.41%	2.57%	3.83%
Defined benefit plan assumptions used calculating net periodic benefit cost, discount rate	2.57%	3.83%	3.19%
Postretirement benefits | Minimum
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan assumptions used calculating benefit obligation, discount rate	1.92%	2.85%	4.03%
Defined benefit plan assumptions used calculating net periodic benefit cost, discount rate	2.87%	4.04%	3.35%
Postretirement benefits | Maximum
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan assumptions used calculating benefit obligation, discount rate	2.81%	3.44%	4.56%
Defined benefit plan assumptions used calculating net periodic benefit cost, discount rate	3.44%	4.56%	3.87%
Qualified pension/ postretirement benefits
Defined Benefit Plan Disclosure [Line Items]
Expected time horizon	30 years
Defined benefit plan assumptions used calculating net periodic benefit cost, expected long-term rate of return on plan assets	3.45%	4.80%	4.20%
Qualified pension/ postretirement benefits | Maximum
Defined Benefit Plan Disclosure [Line Items]
Term used for assumption calculation	30 years
Postretirement benefit (retirees post January 1, 1993)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan assumptions used calculating net periodic benefit cost, expected long-term rate of return on plan assets	6.40%	6.85%	5.95%
Postretirement benefit (retirees prior to January 1, 1993)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan assumptions used calculating net periodic benefit cost, expected long-term rate of return on plan assets	0.90%	0.05%	2.15%