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Commitments and Contingencies - Schedule of Company's Restricted Assets (Details) - USD ($) $ in Millions | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Sep. 19, 2018 | May 01, 2018
Fair Value, Separate Account Investment [Line Items] |  |  |  |  |
Restricted assets, excluding illiquid assets | $ 4,306.9 | $ 4,156.9 |  |  |
Other investments | 109.4 | 111.4 |  |  |
Total restricted assets | $ 4,416.3 | $ 4,268.3 |  |  |
Total as percent of investable assets | 58.60% | 54.40% |  |  |
Securities and cash as collateral secured letters of credit | $ 516.8 | $ 635.4 |  |  |
Affiliated transactions |  |  |  |  |
Fair Value, Separate Account Investment [Line Items] |  |  |  |  |
Restricted Assets | 1,027.9 | 754.9 |  |  |
Third party |  |  |  |  |
Fair Value, Separate Account Investment [Line Items] |  |  |  |  |
Restricted Assets | 2,762.2 | 2,766.6 |  |  |
Letters of credit / guarantees |  |  |  |  |
Fair Value, Separate Account Investment [Line Items] |  |  |  |  |
Restricted Assets | 516.8 | 635.4 |  |  |
Limited Partner | Real estate fund |  |  |  |  |
Fair Value, Separate Account Investment [Line Items] |  |  |  |  |
Restricted Assets |  |  | $ 100.0 |  |
Other investments | $ 109.4 | $ 111.4 |  | $ 13.8 | $ 86.2

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