Carrying Amounts and Fair Values of Financial Instruments (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Preferred stock with mandatory redemption requirements, carrying amount	$ 12	$ 13
Long-term debt (excluding capital lease obligations), carrying amount	6,320	5,914
Preferred stock with mandatory redemption requirements, fair value	15	18
Long-term debt (excluding capital lease obligations), fair value	7,044	6,757
Level 1
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Long-term debt (excluding capital lease obligations), fair value	3,876	3,397
Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Long-term debt (excluding capital lease obligations), fair value	1,363	1,419
Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Preferred stock with mandatory redemption requirements, fair value	15	18
Long-term debt (excluding capital lease obligations), fair value	$ 1,805	$ 1,941