Significant Accounting Policies - Reconciliation of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 2,762	$ 1,694	[1]	$ 1,482
Other assets:
Restricted cash	106	105	[1]	91
Total cash, cash equivalents and restricted cash shown in the statement of consolidated cash flows	2,868	1,799	[2]	1,591	[2]	$ 2,303	[2]
Prepaid Expenses and Other
Current assets:
Restricted cash	0	0		18
United Airlines, Inc.
Current assets:
Cash and cash equivalents	2,756	1,688	[3]	1,476
Other assets:
Restricted cash	106	105	[3]	91
Total cash, cash equivalents and restricted cash shown in the statement of consolidated cash flows	2,862	1,793	[4]	1,585	[4]	$ 2,297	[4]
United Airlines, Inc. | Prepaid Expenses and Other
Current assets:
Restricted cash	$ 0	$ 0		$ 18

[1] Amounts adjusted due to the adoption of Accounting Standards Update No. 2016-02, Leases (Topic 842). 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. 2016-02, Leases (Topic 842). See Note 1 to the financial statements contained in Part II, Item 8 of this report for additional information.

[3] Amounts adjusted due to the adoption of Accounting Standards Update No. 2016-02, Leases (Topic 842). See Note 1 to the financial statements contained in Part II, Item 8 of this report for additional information.

[4] Amounts adjusted due to the adoption of Accounting Standards Update No. 2016-02, Leases (Topic 842). See Note 1 to the financial statements contained in Part II, Item 8 of this report for additional information.