Significant Accounting Policies - Reconciliation of Cash, Cash Equivalents and Restricted Cash (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 11,269	$ 2,762	$ 1,694
Restricted cash	255	0	0
Other assets:
Restricted cash	218	106	105
Total cash, cash equivalents and restricted cash shown in the statement of consolidated cash flows	11,742	2,868	1,799	$ 1,591
United Airlines, Inc.
Current assets:
Cash and cash equivalents	11,269	2,756	1,688
Restricted cash	255	0	0
Other assets:
Restricted cash	218	106	105
Total cash, cash equivalents and restricted cash shown in the statement of consolidated cash flows	$ 11,742	$ 2,862	$ 1,793	$ 1,585