Fair Values of Financial Instruments - Carrying Amounts and Fair Values of Financial Instruments (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Preferred stock with mandatory redemption requirements, carrying amount	$ 10	$ 12
Long-term debt (excluding capital lease obligations), carrying amount	6,809	6,320
Preferred stock with mandatory redemption requirements, fair value	14	15
Long-term debt (excluding capital lease obligations), fair value	7,643	7,044
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt (excluding capital lease obligations), fair value	4,846	3,876
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term debt (excluding capital lease obligations), fair value	976	1,363
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Preferred stock with mandatory redemption requirements, fair value	14	15
Long-term debt (excluding capital lease obligations), fair value	$ 1,821	$ 1,805