Fair Value Measurements - Measured on Recurring Basis (Details) - USD ($) $ in Millions		3 Months Ended
		Sep. 30, 2015		Dec. 31, 2012	Dec. 31, 2014
Fuel derivatives:
Derivative Asset, Fair Value, Gross Asset	[1]	$ 1,736			$ 1,360
Fuel derivatives:
Derivative Liability, Fair Value, Gross Liability	[1]	(3,014)			(2,409)
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount		0			0
Fair Value, Liabilities, Level 1 to Level 2 Transfers, Amount		0			0
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount		0			0
Fair Value, Liabilities, Level 2 to Level 1 Transfers, Amount		0			0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers, Net		0		$ 0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Transfers, Net		0		$ 0
Fair Value, Measurements, Recurring | Period end date
Assets
Treasury bills		1,149			1,450
Interest rate derivatives		6			13
Other available for sale securities		62			68
Total assets		4,921			4,443
Liabilities
Interest rate derivatives		(54)			(61)
Total Liabilities		(3,014)			(2,409)
Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1)
Assets
Treasury bills		1,149			1,450
Interest rate derivatives		0			0
Other available for sale securities		62			63
Total assets		2,739			2,623
Liabilities
Interest rate derivatives		0			0
Total Liabilities		0			0
Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2)
Assets
Treasury bills		0			0
Interest rate derivatives		6			13
Other available for sale securities		0			0
Total assets		1,243			896
Liabilities
Interest rate derivatives		(54)			(61)
Total Liabilities		(733)			(426)
Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Assets
Treasury bills		0			0
Interest rate derivatives		0			0
Other available for sale securities		0			5
Total assets		939			924
Liabilities
Interest rate derivatives		0			0
Total Liabilities		(2,281)			(1,983)
Cash Equivalents | Fair Value, Measurements, Recurring | Period end date
Assets
Cash equivalents		1,528	[2]		1,110	[3]
Cash Equivalents | Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1)
Assets
Cash equivalents		1,528	[2]		1,110	[3]
Cash Equivalents | Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2)
Assets
Cash equivalents		0	[2]		0	[3]
Cash Equivalents | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Assets
Cash equivalents		0	[2]		0	[3]
Commercial paper | Fair Value, Measurements, Recurring | Period end date
Assets
Cash equivalents		125			70
Commercial paper | Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1)
Assets
Cash equivalents		0			0
Commercial paper | Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2)
Assets
Cash equivalents		125			70
Commercial paper | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Assets
Cash equivalents		0			0
Certificates of deposit | Fair Value, Measurements, Recurring | Period end date
Assets
Cash equivalents		25			4
Certificates of deposit | Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1)
Assets
Cash equivalents		0			0
Certificates of deposit | Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2)
Assets
Cash equivalents		25			4
Certificates of deposit | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Assets
Cash equivalents		0			0
Eurodollar time deposits | Fair Value, Measurements, Recurring | Period end date
Assets
Cash equivalents		62			98
Eurodollar time deposits | Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1)
Assets
Cash equivalents		0			0
Eurodollar time deposits | Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2)
Assets
Cash equivalents		62			98
Eurodollar time deposits | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Assets
Cash equivalents		0			0
Certificates of deposit | Fair Value, Measurements, Recurring | Period end date
Assets
Investments		207			256
Certificates of deposit | Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1)
Assets
Investments		0			0
Certificates of deposit | Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2)
Assets
Investments		207			256
Certificates of deposit | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Assets
Investments		0			0
Auction rate securities | Fair Value, Measurements, Recurring | Period end date
Assets
Investments		27	[4]		27	[5]
Auction rate securities | Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1)
Assets
Investments		0	[4]		0	[5]
Auction rate securities | Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2)
Assets
Investments		0	[4]		0	[5]
Auction rate securities | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Assets
Investments		27	[4]		27	[5]
Swap | Fair Value, Measurements, Recurring | Period end date
Fuel derivatives:
Derivative Asset, Fair Value, Gross Asset		818	[6]		455	[7]
Fuel derivatives:
Derivative Asset, Fair Value, Gross Liability		(679)	[6]		(365)	[7]
Swap | Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1)
Fuel derivatives:
Derivative Asset, Fair Value, Gross Asset		0	[6]		0	[7]
Fuel derivatives:
Derivative Asset, Fair Value, Gross Liability		0	[6]		0	[7]
Swap | Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2)
Fuel derivatives:
Derivative Asset, Fair Value, Gross Asset		818	[6]		455	[7]
Fuel derivatives:
Derivative Asset, Fair Value, Gross Liability		(679)	[6]		(365)	[7]
Swap | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Fuel derivatives:
Derivative Asset, Fair Value, Gross Asset		0	[6]		0	[7]
Fuel derivatives:
Derivative Asset, Fair Value, Gross Liability		0	[6]		0	[7]
Options Held | Fair Value, Measurements, Recurring | Period end date
Fuel derivatives:
Derivative Asset, Fair Value, Gross Asset		912	[6]		892	[7]
Fuel derivatives:
Derivative Asset, Fair Value, Gross Liability		(2,281)	[6]		(1,983)	[7]
Options Held | Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets (Level 1)
Fuel derivatives:
Derivative Asset, Fair Value, Gross Asset		0	[6]		0	[7]
Fuel derivatives:
Derivative Asset, Fair Value, Gross Liability		0	[6]		0	[7]
Options Held | Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2)
Fuel derivatives:
Derivative Asset, Fair Value, Gross Asset		0	[6]		0	[7]
Fuel derivatives:
Derivative Asset, Fair Value, Gross Liability		0	[6]		0	[7]
Options Held | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Fuel derivatives:
Derivative Asset, Fair Value, Gross Asset		912	[6]		892	[7]
Fuel derivatives:
Derivative Asset, Fair Value, Gross Liability		$ (2,281)	[6]		$ (1,983)	[7]

[1] Represents the position of each trade before consideration of offsetting positions with each counterparty and does not include the impact of cash collateral deposits provided to or received from counterparties. See discussion of credit risk and collateral following in this Note.

[2] Cash equivalents are primarily composed of money market investments.

[3] Cash equivalents are primarily composed of money market investments.

[4] Noncurrent investments are included in Other assets in the unaudited Condensed Consolidated Balance Sheet.

[5] Noncurrent investments are included in Other assets in the unaudited Condensed Consolidated Balance Sheet.

[6] In the unaudited Condensed Consolidated Balance Sheet amounts are presented as a net liability.

[7] In the unaudited Condensed Consolidated Balance Sheet amounts are presented as a net liability.