| Carrying amount and estimated fair value, long-term debt |
The carrying amounts and estimated fair values of the Company's long-term debt at March 31, 2021 and December 31, 2020 were as follows: | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | March 31, 2021 | | December 31, 2020 | | Fair Value Level Hierarchy | | | Carrying Value | | Estimated Fair Value | | Carrying Value | | Estimated Fair Value | | | (in millions) | | | 8.00% senior secured notes | $ | 850.0 | | | $ | 881.8 | | | $ | 850.0 | | | $ | 886.0 | | | Level 3 | |
|
| Fixed-rate term loans | 1,272.9 | | | 1,303.2 | | | 1,301.9 | | | 1,362.9 | | | Level 3 | | Unsecured term loans | 98.7 | | | 105.5 | | | 73.3 | | | 83.1 | | | Level 3 | | 2015-1 EETC Class A | 322.6 | | | 323.4 | | | 322.6 | | | 323.4 | | | Level 2 | | 2015-1 EETC Class B | 64.0 | | | 63.8 | | | 64.0 | | | 62.5 | | | Level 2 | | 2015-1 EETC Class C | 86.6 | | | 81.7 | | | 86.6 | | | 77.8 | | | Level 2 | | 2017-1 EETC Class AA | 207.3 | | | 207.9 | | | 214.4 | | | 207.4 | | | Level 2 | | 2017-1 EETC Class A | 69.1 | | | 67.2 | | | 71.5 | | | 68.8 | | | Level 2 | | 2017-1 EETC Class B | 58.2 | | | 56.2 | | | 60.6 | | | 56.2 | | | Level 2 | | 2017-1 EETC Class C | 85.5 | | | 79.9 | | | 85.5 | | | 76.3 | | | Level 2 | | Convertible debt | 175.0 | | | 538.2 | | | 175.0 | | | 380.3 | | | Level 2 | | Revolving credit facilities | 180.0 | | | 180.0 | | | 275.1 | | | 275.1 | | | Level 3 | | Total long-term debt | $ | 3,469.9 | | | $ | 3,888.8 | | | $ | 3,580.5 | | | $ | 3,859.9 | | | |
|
| Schedule of assets and liabilities measured at fair value on a recurring basis |
Assets and liabilities measured at gross fair value on a recurring basis are summarized below: | | | | | | | | | | | | | | | | | | | | | | | | | | Fair Value Measurements as of March 31, 2021 | | | Total | | Level
1 | | Level
2 | | Level
3 | | (in millions) | | Cash and cash equivalents | $ | 1,774.5 | | | $ | 1,774.5 | | | $ | — | | | $ | — | | |
|
| Restricted cash | 64.0 | | | 64.0 | | | — | | | — | | | Short-term investment securities | 106.4 | | | 106.4 | | | — | | | — | | | Assets held for sale | 2.3 | | | — | | | — | | | 2.3 | | | Total assets | $ | 1,947.2 | | | $ | 1,944.9 | | | $ | — | | | $ | 2.3 | | | | | | | | | | |
|
|
| Total liabilities | $ | — | | | $ | — | | | $ | — | | | $ | — | |
| | | | | | | | | | | | | | | | | | | | | | | | | | Fair Value Measurements as of December 31, 2020 | | | Total | | Level
1 | | Level
2 | | Level
3 | | (in millions) | | Cash and cash equivalents | $ | 1,789.7 | | | $ | 1,789.7 | | | $ | — | | | $ | — | | | Restricted cash | 71.4 | | | 71.4 | | | — | | | — | | |
|
| Short-term investment securities | 106.3 | | | 106.3 | | | — | | | — | | | Assets held for sale | 2.3 | | | — | | | — | | | 2.3 | | | Total assets | $ | 1,969.7 | | | $ | 1,967.4 | | | $ | — | | | $ | 2.3 | | | | | | | | | | |
|
|
| Total liabilities | $ | — | | | $ | — | | | $ | — | | | $ | — | |
|