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Investments - Additional Information (Narrative) (Details) - USD ($) $ in Millions | 1 Months Ended | 12 Months Ended |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Mar. 31, 2021 | Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020 | May 05, 2022 | Apr. 01, 2022 | Sep. 30, 2021 | Dec. 23, 2019 | Dec. 20, 2017 | Jan. 01, 2017
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |
Catastrophe bonds, trading, cost |  |  | $ 2.9 | $ 3.4 |  |  |  |  |  |  |
Investment in the period |  |  | 2.0 | 3.2 |  |  |  |  |  |  |
Equity method investment, aggregate cost |  |  | 6.2 | 3.9 | $ 0.9 |  |  |  |  |  |
Other investments, at fair value (2) | [1] |  | 221.3 | 151.3 |  |  |  |  |  |  |
Restricted assets |  |  | 471.3 | 539.8 |  |  |  |  |  |  |
Investment in Multiline reinsurer |  |  | 0.0 | 0.5 |  |  |  |  |  |  |
Investment funded |  |  | 0.0 |  |  |  |  |  |  |  |
Available-for-sale investment, allowance for credit loss, increase (decrease) |  |  | 5.0 | 2.5 |  |  |  |  |  |  |
Investment, Identifier [Axis]: Unfunded Commitment, Apollo Real Estate Fund |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |
Unfunded commitment |  |  | 5.3 |  |  |  |  |  |  |  |
Investment, Identifier [Axis]: Unfunded Commitment, Infrastructure fund |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |
Unfunded commitment |  |  | 8.4 |  |  |  |  |  |  |  |
Investment, Identifier [Axis]: Unfunded Commitment, Real Estate Fund (Sept 2021) |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |
Unfunded commitment |  |  | 3.3 |  |  |  |  |  |  |  |
Fair Value, Recurring [Member] | Apollo real estate fund |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |
Other investments, at fair value (2) |  |  | 25.3 |  |  |  |  |  |  |  |
Real estate fund | Fair Value, Recurring [Member] |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |
Investments, fair value |  |  | 221.3 | 151.3 |  |  |  |  |  |  |
Commercial mortgage loans |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |
Fair market value, trading |  |  | 312.1 | 211.5 |  |  |  |  |  |  |
Middle market loans |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |
Fair market value, trading |  |  | 106.9 | 65.4 |  |  |  |  |  |  |
Privately-held investments |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |
Fair market value, trading |  |  | 533.0 | 307.1 |  |  |  |  |  |  |
Real estate fund (Dec 2017) | Fair Value, Recurring [Member] |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |
Investments, fair value |  |  | 134.6 | 129.9 |  |  |  |  |  |  |
Real estate fund (Sept 2021) | Fair Value, Recurring [Member] |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |
Investments, fair value |  |  | 40.6 |  |  |  |  |  |  |  |
Infrastructure fund | Fair Value, Recurring [Member] |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |
Investments, fair value |  |  | 8.2 |  |  |  |  |  |  |  |
MVI |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |
Investment in the period |  | $ 0.8 |  | 0.4 |  |  |  |  |  |  |
Equity method investment, aggregate cost |  |  | 0.8 | 0.5 | 0.0 |  |  |  |  |  |
Gross realized and unrealized gains (loss) |  |  | 0.0 | 0.1 | (0.4) |  |  |  |  |  |
Investment in Multiline reinsurer |  |  | 0.0 | 0.0 |  |  |  |  |  |  |
Digital Re |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |
Investment in the period |  |  | 0.0 | 0.3 |  |  |  |  |  |  |
Equity method investment, ownership percentage |  |  |  |  |  |  |  |  |  |  | 49.00%
Equity method investment, aggregate cost |  |  | 0.2 | 0.2 | 0.4 |  |  |  |  |  | $ 2.3
Gross realized and unrealized gains (loss) |  |  | 0.0 | 0.0 | (0.3) |  |  |  |  |  |
Investment in Multiline reinsurer |  |  | 0.0 | 0.5 |  |  |  |  |  |  |
Multi-Line Insurer |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |
Investment in the period |  |  | 1.6 | 2.5 |  |  |  |  |  |  |
Equity method investment, aggregate cost |  |  | 5.2 | 3.2 | 0.5 |  |  |  |  |  |
Other investments, at fair value (2) |  |  |  |  |  |  |  |  | $ 5.0 |  |
Gross realized and unrealized gains (loss) |  |  | (0.4) | 0.2 | $ 0.0 |  |  |  |  |  |
Investment in Multiline reinsurer |  |  | 0.0 | 0.0 |  |  |  |  |  |  |
Limited Partner | Real estate fund |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |
Other investments, at fair value (2) |  |  | $ 221.3 | $ 151.3 |  |  |  |  |  |  |
Restricted assets |  |  |  |  |  |  | $ 10.0 | $ 20.0 |  | $ 100.0 |
Limited Partner | Real estate fund | Apollo real estate fund |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |
Restricted assets |  |  |  |  |  |  | $ 30.0 |  |  |  |
Limited Partner | Infrastructure fund |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |
Restricted assets |  |  |  |  |  | $ 15.0 |  |  |  |  |
[1] | Other investments includes related party investments of $25.3 million in Apollo Real Estate Fund and $12.7 million in Apollo Origination Partnership. (3) Cash and cash equivalents includes restricted cash of $232.1 million (2021 — $364.9 million) which are held in trusts.

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