Financial Instruments and Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended	9 Months Ended
	Apr. 30, 2018	Sep. 30, 2018	Sep. 30, 2017
Fair Value [Line Items]
Amount invested		$ 1,975	$ 2,584
Azul
Fair Value [Line Items]
Amount invested	$ 138
Equity stake percentage	2.00%
Azul | Preferred Stock
Fair Value [Line Items]
Equity stake percentage	8.00%
Minimum | Asset-backed Securities
Fair Value [Line Items]
Available-for-sale securities remaining maturities (less than one year)		1 year
Minimum | Corporate Debt
Fair Value [Line Items]
Available-for-sale securities remaining maturities (less than one year)		1 year
Minimum | U.S. Government and Other Securities
Fair Value [Line Items]
Available-for-sale securities remaining maturities (less than one year)		1 year
Maximum | Asset-backed Securities
Fair Value [Line Items]
Available-for-sale securities remaining maturities (less than one year)		16 years
Maximum | Corporate Debt
Fair Value [Line Items]
Available-for-sale securities remaining maturities (less than one year)		3 years
Maximum | CDARS
Fair Value [Line Items]
Available-for-sale securities remaining maturities (less than one year)		1 year
Maximum | U.S. Government and Other Securities
Fair Value [Line Items]
Available-for-sale securities remaining maturities (less than one year)		2 years