Fair Value of Financial Assets and Liabilities - Carrying Amounts and Fair Values of Financial Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Preferred stock with mandatory redemption requirements	$ 15	$ 15
Long-term debt (excluding capital lease obligations), fair value	8,232	7,044
Fair Value | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Preferred stock with mandatory redemption requirements	0	0
Long-term debt (excluding capital lease obligations), fair value	5,375	3,876
Fair Value | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Preferred stock with mandatory redemption requirements	0	0
Long-term debt (excluding capital lease obligations), fair value	990	1,363
Fair Value | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Preferred stock with mandatory redemption requirements	15	15
Long-term debt (excluding capital lease obligations), fair value	1,867	1,805
Carrying Amount
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Preferred stock with mandatory redemption requirements	11	12
Long-term debt (excluding capital lease obligations), carrying amount	$ 7,357	$ 6,320