Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2016

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, 2016									Valuation Technique
(in millions)	Total			Level 1			Level 2
Cash equivalents	$	2,279		$	2,279		$	—		(a)
Short-term investments
U.S. government and agency 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)
Interest rate contract	6			—			6			(a)(b)
Foreign currency exchange contracts	27			—			27			(a)

	December 31, 2015								Valuation Technique
(in millions)	Total			Level 1		Level 2
Cash equivalents	$	1,543		$	1,543	$	—		(a)
Short-term investments
U.S. government securities	151			74		77			(a)
Asset- and mortgage-backed securities	380			—		380			(a)
Corporate obligations	896			—		896			(a)
Other fixed income securities	38			—		38			(a)
Restricted cash equivalents and investments	49			49		—			(a)
Long-term investments	155			130		25			(a)
Hedge derivatives, net
Fuel hedge contracts	(672		)	65		(737		)	(a)(b)
Interest rate contract	(3		)	—		(3		)	(a)(b)
Foreign currency exchange contracts	94			—		94			(a)

⁽¹⁾  See Note 8, "Employee Benefit Plans," for fair value of benefit plan assets.