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