Fair Value of Financial Assets and Liabilities - Carrying Amounts and Fair Values of Financial Instruments (Details) - USD ($)
$ in Millions |
Mar. 31, 2018 |
Dec. 31, 2017 |
| Carrying Amount |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Preferred stock with mandatory redemption requirements, carrying amount |
$ 9
|
$ 10
|
| Long-term debt (excluding capital lease obligations), carrying amount |
6,814
|
6,809
|
| Fair Value |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Preferred stock with mandatory redemption requirements, fair value |
12
|
14
|
| Long-term debt (excluding capital lease obligations), fair value |
7,371
|
7,643
|
| Fair Value | 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 capital lease obligations), fair value |
4,630
|
4,846
|
| Fair Value | 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 capital lease obligations), fair value |
959
|
976
|
| Fair Value | Level 3 |
|
|
| Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |
|
|
| Preferred stock with mandatory redemption requirements, fair value |
12
|
14
|
| Long-term debt (excluding capital lease obligations), fair value |
$ 1,782
|
$ 1,821
|