Fair Values of Financial Instruments Fair Values of Financial Instruments - Carrying Amounts and Fair Values of Financial Instruments (Details) - USD ($)
$ in Millions |
Dec. 31, 2019 |
Dec. 31, 2018 |
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Preferred stock with mandatory redemption requirements, carrying amount |
$ 7
|
$ 8
|
| Long-term debt (excluding finance lease obligations), carrying amount |
8,664
|
7,638
|
| Preferred stock with mandatory redemption requirements, fair value |
9
|
9
|
| Long-term debt (excluding finance lease obligations), fair value |
9,770
|
7,921
|
| Level 1 |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Preferred stock with mandatory redemption requirements, fair value |
0
|
0
|
| Long-term debt (excluding finance lease obligations), fair value |
7,689
|
5,760
|
| Level 2 |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Preferred stock with mandatory redemption requirements, fair value |
0
|
0
|
| Long-term debt (excluding finance lease obligations), fair value |
417
|
433
|
| Level 3 |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Preferred stock with mandatory redemption requirements, fair value |
9
|
9
|
| Long-term debt (excluding finance lease obligations), fair value |
$ 1,664
|
$ 1,728
|