Significant Accounting Policies - Reconciliation of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Current assets:
Cash and cash equivalents	$ 1,694	$ 1,482	[1]	$ 2,179
Other assets:
Restricted cash	105	91	[1]	124
Total cash, cash equivalents and restricted cash shown in the statement of consolidated cash flows	1,799	1,591	[2]	2,303	[2]	$ 3,212	[2]
Prepaid Expenses and Other
Current assets:
Restricted cash	0	18		0
United Airlines, Inc.
Current assets:
Cash and cash equivalents	1,688	1,476	[3]	2,173
Other assets:
Restricted cash	105	91	[3]	124
Total cash, cash equivalents and restricted cash shown in the statement of consolidated cash flows	1,793	1,585	[4]	2,297	[4]	$ 3,206	[4]
United Airlines, Inc. | Prepaid Expenses and Other
Current assets:
Restricted cash	$ 0	$ 18		$ 0

[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.

[3] 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.

[4] 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.