Fair Value Measurements - Fair Value of Long-Term Fixed Rate and Variable Rate Debt Excluding Capital Leases (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount — liability	$ 5,799	$ 5,193
Fixed Rate Debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount — liability	3,586	3,514
Fair value — liability	3,773	3,669
Variable Rate Debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount — liability	2,213	1,679
Fair value — liability	$ 2,198	$ 1,678