Investments and Fair Value Measurements - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	[1]	Dec. 31, 2016	[1]	Jan. 31, 2019	Nov. 29, 2018
Fair Value [Line Items]
Investment in equity securities	$ 2,552.0	$ 3,241.0		$ 2,768.0
Investments in securities accounted for under the equity method	$ 144.0
EETC
Fair Value [Line Items]
Available-for-sale securities remaining maturities	2019
Asset-backed Securities | Minimum
Fair Value [Line Items]
Available-for-sale securities remaining maturities	1 year
Asset-backed Securities | Maximum
Fair Value [Line Items]
Available-for-sale securities remaining maturities	16 years
Corporate Debt | Minimum
Fair Value [Line Items]
Available-for-sale securities remaining maturities	1 year
Corporate Debt | Maximum
Fair Value [Line Items]
Available-for-sale securities remaining maturities	3 years
CDARS | Maximum
Fair Value [Line Items]
Available-for-sale securities remaining maturities	1 year
U.S. Government and Other Securities | Minimum
Fair Value [Line Items]
Available-for-sale securities remaining maturities	1 year
Azul
Fair Value [Line Items]
Investment in equity securities	$ 138.0
Equity stake	2.00%
Azul | Preferred Stock
Fair Value [Line Items]
Equity stake	8.00%
Republic
Fair Value [Line Items]
Ownership stake	19.00%
ManaAir | Subsequent Event
Fair Value [Line Items]
Ownership stake						49.90%
ExpressJet Airlines, Inc. | ManaAir | Subsequent Event
Fair Value [Line Items]
Ownership stake						100.00%
Champlain
Fair Value [Line Items]
Ownership stake	40.00%
Fulcrum
Fair Value [Line Items]
Equity Securities Without Readily Determinable Fair Value, Ownership Interest	0.09
Carrying value of other investment	$ 48.0
Synergy
Fair Value [Line Items]
Loan amount							$ 456.0
Number of shares that may be put to United (in shares)							516,000,000
Number of ADRs that may be put to United (in shares)							64,500,000
Percentage of installment that may be paid in shares (up to)							25.00%
Annual interest rate							3.00%
Number of shares that may be purchased at United's option (up to) (in shares)							77,400,000.0
Shares subject to increase in value agreement (in shares)							438,600,000.0
Kingsland
Fair Value [Line Items]
Number of shares that may be put to United (in shares)							144,800,000.0
Number of ADRs that may be put to United (in shares)							18,100,000
Market price of common stock at fifth anniversary (in dollars per share)							$ 12
Aggregate maximum possible combined put payment and guarantee amount							$ 217.2

[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.