Retirement Benefits - Actuarial Assumptions (Details)
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6 Months Ended |
12 Months Ended |
Dec. 31, 2016 |
Jun. 30, 2016 |
Dec. 31, 2018 |
Dec. 31, 2017 |
Dec. 31, 2016 |
| Pension plans |
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| Defined Benefit Plan Disclosure [Line Items] |
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| Discount rate – benefit obligations |
4.58%
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|
3.80%
|
4.40%
|
4.58%
|
| Expected long-term return on plan assets |
|
|
7.95%
|
7.92%
|
8.16%
|
| Annual salary increase, net periodic costs |
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|
4.34%
|
4.37%
|
4.37%
|
| Pension plans | Discount rate – interest costs |
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| Defined Benefit Plan Disclosure [Line Items] |
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| Discount rate, net periodic benefit costs |
|
|
3.45%
|
3.77%
|
3.88%
|
| Pension plans | Discount rate – service costs |
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| Defined Benefit Plan Disclosure [Line Items] |
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| Discount rate, net periodic benefit costs |
|
|
3.98%
|
4.81%
|
4.98%
|
| Pension plans | Employee benefit obligations |
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| Defined Benefit Plan Disclosure [Line Items] |
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| Discount rate – benefit obligations |
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|
4.49%
|
3.80%
|
|
| Annual salary increase |
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|
4.34%
|
4.32%
|
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| Other postretirement benefit plans |
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| Defined Benefit Plan Disclosure [Line Items] |
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| Discount rate – benefit obligations |
4.38%
|
|
3.68%
|
4.23%
|
4.38%
|
| Expected long-term return on plan assets |
|
|
6.83%
|
6.84%
|
6.66%
|
| Annual salary increase, net periodic costs |
|
|
4.34%
|
4.37%
|
4.37%
|
| Other postretirement benefit plans | Discount rate – interest costs |
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|
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| Defined Benefit Plan Disclosure [Line Items] |
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|
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| Discount rate, net periodic benefit costs |
|
|
3.29%
|
3.54%
|
3.66%
|
| Other postretirement benefit plans | Discount rate – service costs |
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| Defined Benefit Plan Disclosure [Line Items] |
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|
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| Discount rate, net periodic benefit costs |
|
|
3.91%
|
4.64%
|
4.85%
|
| Other postretirement benefit plans | Employee benefit obligations |
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| Defined Benefit Plan Disclosure [Line Items] |
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| Discount rate – benefit obligations |
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|
4.37%
|
3.68%
|
|
| Annual salary increase |
|
|
4.34%
|
4.32%
|
|
| Alabama Power | Pension plans |
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| Defined Benefit Plan Disclosure [Line Items] |
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| Discount rate – benefit obligations |
4.67%
|
|
3.81%
|
4.44%
|
4.67%
|
| Expected long-term return on plan assets |
|
|
7.95%
|
7.95%
|
8.20%
|
| Annual salary increase, net periodic costs |
|
|
4.46%
|
4.46%
|
4.46%
|
| Alabama Power | Pension plans | Discount rate – interest costs |
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| Defined Benefit Plan Disclosure [Line Items] |
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| Discount rate, net periodic benefit costs |
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|
3.45%
|
3.76%
|
3.90%
|
| Alabama Power | Pension plans | Discount rate – service costs |
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| Defined Benefit Plan Disclosure [Line Items] |
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| Discount rate, net periodic benefit costs |
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|
4.00%
|
4.85%
|
5.07%
|
| Alabama Power | Pension plans | Employee benefit obligations |
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| Defined Benefit Plan Disclosure [Line Items] |
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| Discount rate – benefit obligations |
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|
4.51%
|
3.81%
|
|
| Annual salary increase |
|
|
4.46%
|
4.46%
|
|
| Alabama Power | Other postretirement benefit plans |
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| Defined Benefit Plan Disclosure [Line Items] |
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| Discount rate – benefit obligations |
4.51%
|
|
3.71%
|
4.27%
|
4.51%
|
| Expected long-term return on plan assets |
|
|
6.83%
|
6.83%
|
6.83%
|
| Annual salary increase, net periodic costs |
|
|
4.46%
|
4.46%
|
4.46%
|
| Alabama Power | Other postretirement benefit plans | Discount rate – interest costs |
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| Defined Benefit Plan Disclosure [Line Items] |
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| Discount rate, net periodic benefit costs |
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|
3.31%
|
3.58%
|
3.69%
|
| Alabama Power | Other postretirement benefit plans | Discount rate – service costs |
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| Defined Benefit Plan Disclosure [Line Items] |
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| Discount rate, net periodic benefit costs |
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|
3.93%
|
4.70%
|
4.96%
|
| Alabama Power | Other postretirement benefit plans | Employee benefit obligations |
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| Defined Benefit Plan Disclosure [Line Items] |
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| Discount rate – benefit obligations |
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|
4.40%
|
3.71%
|
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| Annual salary increase |
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|
4.46%
|
4.46%
|
|
| Georgia Power | Pension plans |
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| Defined Benefit Plan Disclosure [Line Items] |
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| Discount rate – benefit obligations |
4.65%
|
|
3.79%
|
4.40%
|
4.65%
|
| Expected long-term return on plan assets |
|
|
7.95%
|
7.95%
|
8.20%
|
| Annual salary increase, net periodic costs |
|
|
4.46%
|
4.46%
|
4.46%
|
| Georgia Power | Pension plans | Discount rate – interest costs |
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| Defined Benefit Plan Disclosure [Line Items] |
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| Discount rate, net periodic benefit costs |
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|
3.42%
|
3.72%
|
3.86%
|
| Georgia Power | Pension plans | Discount rate – service costs |
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| Defined Benefit Plan Disclosure [Line Items] |
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| Discount rate, net periodic benefit costs |
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|
3.99%
|
4.83%
|
5.03%
|
| Georgia Power | Pension plans | Employee benefit obligations |
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| Defined Benefit Plan Disclosure [Line Items] |
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| Discount rate – benefit obligations |
|
|
4.48%
|
3.79%
|
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| Annual salary increase |
|
|
4.46%
|
4.46%
|
|
| Georgia Power | Other postretirement benefit plans |
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| Defined Benefit Plan Disclosure [Line Items] |
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| Discount rate – benefit obligations |
4.49%
|
|
3.68%
|
4.23%
|
4.49%
|
| Expected long-term return on plan assets |
|
|
6.80%
|
6.79%
|
6.27%
|
| Annual salary increase, net periodic costs |
|
|
4.46%
|
4.46%
|
4.46%
|
| Georgia Power | Other postretirement benefit plans | Discount rate – interest costs |
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| Defined Benefit Plan Disclosure [Line Items] |
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| Discount rate, net periodic benefit costs |
|
|
3.29%
|
3.55%
|
3.67%
|
| Georgia Power | Other postretirement benefit plans | Discount rate – service costs |
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| Defined Benefit Plan Disclosure [Line Items] |
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| Discount rate, net periodic benefit costs |
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|
3.91%
|
4.63%
|
4.88%
|
| Georgia Power | Other postretirement benefit plans | Employee benefit obligations |
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| Defined Benefit Plan Disclosure [Line Items] |
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| Discount rate – benefit obligations |
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|
4.36%
|
3.68%
|
|
| Annual salary increase |
|
|
4.46%
|
4.46%
|
|
| Mississippi Power | Pension plans |
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| Defined Benefit Plan Disclosure [Line Items] |
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| Discount rate – benefit obligations |
4.69%
|
|
3.80%
|
4.44%
|
4.69%
|
| Expected long-term return on plan assets |
|
|
7.95%
|
7.95%
|
8.20%
|
| Annual salary increase, net periodic costs |
|
|
4.46%
|
4.46%
|
4.46%
|
| Mississippi Power | Pension plans | Discount rate – interest costs |
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|
|
|
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| Defined Benefit Plan Disclosure [Line Items] |
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| Discount rate, net periodic benefit costs |
|
|
3.46%
|
3.81%
|
3.97%
|
| Mississippi Power | Pension plans | Discount rate – service costs |
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| Defined Benefit Plan Disclosure [Line Items] |
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| Discount rate, net periodic benefit costs |
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|
3.99%
|
4.83%
|
5.04%
|
| Mississippi Power | Pension plans | Employee benefit obligations |
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| Defined Benefit Plan Disclosure [Line Items] |
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| Discount rate – benefit obligations |
|
|
4.49%
|
3.80%
|
|
| Annual salary increase |
|
|
4.46%
|
4.46%
|
|
| Mississippi Power | Other postretirement benefit plans |
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| Defined Benefit Plan Disclosure [Line Items] |
|
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| Discount rate – benefit obligations |
4.47%
|
|
3.68%
|
4.22%
|
4.47%
|
| Expected long-term return on plan assets |
|
|
6.99%
|
6.88%
|
7.07%
|
| Annual salary increase, net periodic costs |
|
|
4.46%
|
4.46%
|
4.46%
|
| Mississippi Power | Other postretirement benefit plans | Discount rate – interest costs |
|
|
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| Defined Benefit Plan Disclosure [Line Items] |
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|
|
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| Discount rate, net periodic benefit costs |
|
|
3.29%
|
3.55%
|
3.66%
|
| Mississippi Power | Other postretirement benefit plans | Discount rate – service costs |
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| Defined Benefit Plan Disclosure [Line Items] |
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|
|
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| Discount rate, net periodic benefit costs |
|
|
3.91%
|
4.65%
|
4.88%
|
| Mississippi Power | Other postretirement benefit plans | Employee benefit obligations |
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|
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| Defined Benefit Plan Disclosure [Line Items] |
|
|
|
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| Discount rate – benefit obligations |
|
|
4.35%
|
3.68%
|
|
| Annual salary increase |
|
|
4.46%
|
4.46%
|
|
| Southern Power | Pension plans |
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|
|
|
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| Defined Benefit Plan Disclosure [Line Items] |
|
|
|
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| Discount rate – benefit obligations |
|
|
3.94%
|
|
|
| Expected long-term return on plan assets |
|
|
7.95%
|
|
|
| Annual salary increase, net periodic costs |
|
|
4.46%
|
|
|
| Southern Power | Pension plans | Discount rate – interest costs |
|
|
|
|
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| Defined Benefit Plan Disclosure [Line Items] |
|
|
|
|
|
| Discount rate, net periodic benefit costs |
|
|
3.69%
|
|
|
| Southern Power | Pension plans | Discount rate – service costs |
|
|
|
|
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| Defined Benefit Plan Disclosure [Line Items] |
|
|
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| Discount rate, net periodic benefit costs |
|
|
4.01%
|
|
|
| Southern Power | Pension plans | Employee benefit obligations |
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|
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| Defined Benefit Plan Disclosure [Line Items] |
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|
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|
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| Discount rate – benefit obligations |
|
|
4.65%
|
3.94%
|
|
| Annual salary increase |
|
|
4.46%
|
4.46%
|
|
| Southern Power | Other postretirement benefit plans |
|
|
|
|
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| Defined Benefit Plan Disclosure [Line Items] |
|
|
|
|
|
| Discount rate – benefit obligations |
|
|
3.81%
|
|
|
| Expected long-term return on plan assets |
|
|
0.00%
|
|
|
| Annual salary increase, net periodic costs |
|
|
4.46%
|
|
|
| Southern Power | Other postretirement benefit plans | Discount rate – interest costs |
|
|
|
|
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| Defined Benefit Plan Disclosure [Line Items] |
|
|
|
|
|
| Discount rate, net periodic benefit costs |
|
|
3.47%
|
|
|
| Southern Power | Other postretirement benefit plans | Discount rate – service costs |
|
|
|
|
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| Defined Benefit Plan Disclosure [Line Items] |
|
|
|
|
|
| Discount rate, net periodic benefit costs |
|
|
3.93%
|
|
|
| Southern Power | Other postretirement benefit plans | Employee benefit obligations |
|
|
|
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| Defined Benefit Plan Disclosure [Line Items] |
|
|
|
|
|
| Discount rate – benefit obligations |
|
|
4.50%
|
3.81%
|
|
| Annual salary increase |
|
|
4.46%
|
4.46%
|
|
| Southern Company Gas | Pension plans | Successor |
|
|
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| Defined Benefit Plan Disclosure [Line Items] |
|
|
|
|
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| Discount rate, net periodic benefit costs |
3.85%
|
|
3.74%
|
4.39%
|
|
| Expected long-term return on plan assets |
7.75%
|
|
7.95%
|
7.60%
|
|
| Annual salary increase, net periodic costs |
3.50%
|
|
3.07%
|
3.50%
|
|
| Pension band increase |
2.00%
|
|
|
|
|
| Southern Company Gas | Pension plans | Predecessor |
|
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|
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| Defined Benefit Plan Disclosure [Line Items] |
|
|
|
|
|
| Discount rate, net periodic benefit costs |
|
4.60%
|
|
|
|
| Expected long-term return on plan assets |
|
7.80%
|
|
|
|
| Annual salary increase, net periodic costs |
|
3.70%
|
|
|
|
| Pension band increase |
|
2.00%
|
|
|
|
| Southern Company Gas | Pension plans | Discount rate – interest costs | Successor |
|
|
|
|
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| Defined Benefit Plan Disclosure [Line Items] |
|
|
|
|
|
| Discount rate, net periodic benefit costs |
3.21%
|
|
3.41%
|
3.76%
|
|
| Southern Company Gas | Pension plans | Discount rate – interest costs | Predecessor |
|
|
|
|
|
| Defined Benefit Plan Disclosure [Line Items] |
|
|
|
|
|
| Discount rate, net periodic benefit costs |
|
4.00%
|
|
|
|
| Southern Company Gas | Pension plans | Discount rate – service costs | Successor |
|
|
|
|
|
| Defined Benefit Plan Disclosure [Line Items] |
|
|
|
|
|
| Discount rate, net periodic benefit costs |
4.07%
|
|
3.84%
|
4.64%
|
|
| Southern Company Gas | Pension plans | Discount rate – service costs | Predecessor |
|
|
|
|
|
| Defined Benefit Plan Disclosure [Line Items] |
|
|
|
|
|
| Discount rate, net periodic benefit costs |
|
4.80%
|
|
|
|
| Southern Company Gas | Pension plans | Employee benefit obligations |
|
|
|
|
|
| Defined Benefit Plan Disclosure [Line Items] |
|
|
|
|
|
| Discount rate – benefit obligations |
|
|
4.47%
|
3.74%
|
|
| Annual salary increase |
|
|
3.07%
|
2.88%
|
|
| Southern Company Gas | Other postretirement benefit plans | Successor |
|
|
|
|
|
| Defined Benefit Plan Disclosure [Line Items] |
|
|
|
|
|
| Discount rate, net periodic benefit costs |
3.61%
|
|
3.62%
|
4.15%
|
|
| Expected long-term return on plan assets |
5.93%
|
|
5.89%
|
6.03%
|
|
| Annual salary increase, net periodic costs |
3.50%
|
|
3.07%
|
3.50%
|
|
| Southern Company Gas | Other postretirement benefit plans | Predecessor |
|
|
|
|
|
| Defined Benefit Plan Disclosure [Line Items] |
|
|
|
|
|
| Discount rate, net periodic benefit costs |
|
4.40%
|
|
|
|
| Expected long-term return on plan assets |
|
6.60%
|
|
|
|
| Annual salary increase, net periodic costs |
|
3.70%
|
|
|
|
| Southern Company Gas | Other postretirement benefit plans | Discount rate – interest costs | Successor |
|
|
|
|
|
| Defined Benefit Plan Disclosure [Line Items] |
|
|
|
|
|
| Discount rate, net periodic benefit costs |
2.84%
|
|
3.21%
|
3.40%
|
|
| Southern Company Gas | Other postretirement benefit plans | Discount rate – interest costs | Predecessor |
|
|
|
|
|
| Defined Benefit Plan Disclosure [Line Items] |
|
|
|
|
|
| Discount rate, net periodic benefit costs |
|
3.60%
|
|
|
|
| Southern Company Gas | Other postretirement benefit plans | Discount rate – service costs | Successor |
|
|
|
|
|
| Defined Benefit Plan Disclosure [Line Items] |
|
|
|
|
|
| Discount rate, net periodic benefit costs |
3.96%
|
|
3.82%
|
4.55%
|
|
| Southern Company Gas | Other postretirement benefit plans | Discount rate – service costs | Predecessor |
|
|
|
|
|
| Defined Benefit Plan Disclosure [Line Items] |
|
|
|
|
|
| Discount rate, net periodic benefit costs |
|
4.70%
|
|
|
|
| Southern Company Gas | Other postretirement benefit plans | Employee benefit obligations |
|
|
|
|
|
| Defined Benefit Plan Disclosure [Line Items] |
|
|
|
|
|
| Discount rate – benefit obligations |
|
|
4.32%
|
3.62%
|
|
| Annual salary increase |
|
|
3.07%
|
2.56%
|
|