Debt - Summary of Long-Term Debt (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017		Sep. 30, 2017	Jan. 31, 2017
Debt Instrument [Line Items]
Less: current portion of long-term debt	$ (1,230)	$ (1,565)	[1]
Long-term debt, net	12,215	11,703	[1]
United Airlines, Inc.
Debt Instrument [Line Items]
Less: current portion of long-term debt	(1,230)	(1,565)	[2]
Long-term debt, net	12,215	11,703	[2]
UAL and United
Debt Instrument [Line Items]
Long term debt	13,636	13,431
Less: unamortized debt discount, premiums and debt issuance costs	(191)	(163)
Less: current portion of long-term debt	(1,230)	(1,565)
Long-term debt, net	$ 12,215	11,703
6.375% Senior Notes due 2018
Debt Instrument [Line Items]
Fixed interest rate	6.375%
6% Senior Notes due 2020
Debt Instrument [Line Items]
Fixed interest rate	6.00%
4.25% Senior Notes due 2022
Debt Instrument [Line Items]
Fixed interest rate	4.25%
4.25% Senior Notes due 2022 | United Airlines, Inc.
Debt Instrument [Line Items]
Fixed interest rate				4.25%
5% Senior Notes due 2024
Debt Instrument [Line Items]
Fixed interest rate	5.00%
5% Senior Notes due 2024 | United Airlines, Inc.
Debt Instrument [Line Items]
Fixed interest rate					5.00%
Secured Debt | Notes payable, fixed interest rates of 0.0% to 9.52% (weighted average rate of 4.18% as of December 31, 2018), payable through 2030 | United Airlines, Inc.
Debt Instrument [Line Items]
Long term debt	$ 8,811	8,661
Weighted average rate	4.18%
Maturity date	2030
Secured Debt | Notes payable, fixed interest rates of 0.0% to 9.52% (weighted average rate of 4.18% as of December 31, 2018), payable through 2030 | Minimum | United Airlines, Inc.
Debt Instrument [Line Items]
Fixed interest rate	0.00%
Secured Debt | Notes payable, fixed interest rates of 0.0% to 9.52% (weighted average rate of 4.18% as of December 31, 2018), payable through 2030 | Maximum | United Airlines, Inc.
Debt Instrument [Line Items]
Fixed interest rate	9.52%
Secured Debt | Notes payable, floating interest rates of the London interbank offered rate (LIBOR) plus 1.05% to 1.75%, payable through 2030 | United Airlines, Inc.
Debt Instrument [Line Items]
Long term debt	$ 2,051	1,880
Secured Debt | Notes payable, floating interest rates of the London interbank offered rate (LIBOR) plus 1.05% to 1.75%, payable through 2030 | Maximum | United Airlines, Inc.
Debt Instrument [Line Items]
Maturity date	2030
Secured Debt | Notes payable, floating interest rates of the London interbank offered rate (LIBOR) plus 1.05% to 1.75%, payable through 2030 | LIBOR | Minimum | United Airlines, Inc.
Debt Instrument [Line Items]
Basis spread on variable rate	1.05%
Secured Debt | Notes payable, floating interest rates of the London interbank offered rate (LIBOR) plus 1.05% to 1.75%, payable through 2030 | LIBOR | Maximum | United Airlines, Inc.
Debt Instrument [Line Items]
Basis spread on variable rate	1.75%
Secured Debt | Term loan, LIBOR plus 1.75%, or alternative rate based on certain market rates plus 0.75%, due 2024 | United Airlines, Inc.
Debt Instrument [Line Items]
Long term debt	$ 1,474	1,489
Maturity date	2024
Secured Debt | Term loan, LIBOR plus 1.75%, or alternative rate based on certain market rates plus 0.75%, due 2024 | LIBOR | United Airlines, Inc.
Debt Instrument [Line Items]
Basis spread on variable rate	1.75%
Secured Debt | Term loan, LIBOR plus 1.75%, or alternative rate based on certain market rates plus 0.75%, due 2024 | Alternative Rate Based on Certain Market Interest Rates | United Airlines, Inc.
Debt Instrument [Line Items]
Basis spread on variable rate	0.75%
Unsecured Debt | 6.375% Senior Notes due 2018
Debt Instrument [Line Items]
Long term debt	$ 0	300
Fixed interest rate	6.375%
Maturity date	2018
Unsecured Debt | 6% Senior Notes due 2020
Debt Instrument [Line Items]
Long term debt	$ 300	300
Fixed interest rate	6.00%
Maturity date	2020
Unsecured Debt | 4.25% Senior Notes due 2022 | United Airlines, Inc.
Debt Instrument [Line Items]
Long term debt	$ 400	400
Fixed interest rate	4.25%
Maturity date	2022
Unsecured Debt | 5% Senior Notes due 2024 | United Airlines, Inc.
Debt Instrument [Line Items]
Long term debt	$ 300	300
Fixed interest rate	5.00%
Maturity date	2024
Unsecured Debt | Other | United Airlines, Inc.
Debt Instrument [Line Items]
Long term debt	$ 300	$ 101

[1] Amounts adjusted due to the adoption of Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606). See Note 1 to the financial statements contained in Part II, Item 8 of this report for additional information.

[2] Amounts adjusted due to the adoption of Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606). See Note 1 to the financial statements contained in Part II, Item 8 of this report for additional information.