Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2017

Fair Value Disclosures [Abstract]

Schedule of assets (liabilities) measured at fair value on a recurring basis

Assets (Liabilities) Measured at Fair Value on a Recurring Basis⁽¹⁾ 

	December 31, 2017									Valuation Technique
(in millions)	Total			Level 1			Level 2
Cash equivalents	$	1,357		$	1,357		$	—		(a)
Short-term investments
U.S. government and agency securities	93			84			9			(a)
Asset- and mortgage-backed securities	173			—			173			(a)
Corporate obligations	467			—			467			(a)
Other fixed income securities	92			—			92			(a)
Restricted cash equivalents and investments	38			38			—			(a)
Long-term investments	513			485			28			(a)
Hedge derivatives, net
Fuel hedge contracts	(66		)	(43		)	(23		)	(a)(b)
Foreign currency exchange contracts	(17		)	—			(17		)	(a)

	December 31, 2016									Valuation Technique
(in millions)	Total			Level 1			Level 2
Cash equivalents	$	2,279		$	2,279		$	—		(a)
Short-term investments
U.S. government securities	112			86			26			(a)
Asset- and mortgage-backed securities	68			—			68			(a)
Corporate obligations	295			—			295			(a)
Other fixed income securities	12			—			12			(a)
Restricted cash equivalents and investments	61			61			—			(a)
Long-term investments	139			115			24			(a)
Hedge derivatives, net
Fuel hedge contracts	(324		)	(26		)	(298		)	(a)(b)
Foreign currency exchange contracts	27			—			27			(a)

⁽¹⁾  See Note 9, "Employee Benefit Plans," for fair value of benefit plan assets.