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Investments - Additional Information (Narrative) (Details) - USD ($) $ in Millions |  |  | 1 Months Ended | 12 Months Ended |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Dec. 14, 2020 | Jul. 26, 2016 | Jan. 31, 2015 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 27, 2019 | Dec. 23, 2019 | Sep. 19, 2018 | May 01, 2018 | Dec. 18, 2017 | Jan. 01, 2017
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Catastrophe bonds, trading, cost |  |  |  | $ 18.8 | $ 28.6 |  |  |  |  |  |  |  |
Investment in the period |  |  |  | 0.5 | 1.1 |  |  |  |  |  |  |  |
Investment in Multiline reinsurer |  |  |  | 77.2 |  |  |  |  |  |  |  |  |
Equity method investment, aggregate cost |  |  |  | 0.9 | 67.9 | $ 67.1 |  |  |  |  |  |  |
Equity method investment, realized gain (loss) on disposal |  |  |  | 10.4 |  |  |  |  |  |  |  |  |
Other investments |  |  |  | 109.4 | 111.4 |  |  |  |  |  |  |  |
Investment funded |  |  |  | 0.0 |  |  |  |  |  |  |  |  |
Other-than-temporary impairment charge |  |  |  |  | 0.0 |  |  |  |  |  |  |  |
Available-for-sale investment, allowance for credit loss, increase (decrease) |  |  |  | $ (0.4) |  |  |  |  |  |  |  |  |
BBB Emerging Market Debt |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of investment |  |  |  | 0.00% |  |  |  |  |  |  |  |  |
Real estate fund |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of investment |  |  |  | 1.50% |  |  |  |  |  |  |  |  |
Real estate fund | Fair Value, Recurring [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Investments, fair value |  |  |  | $ 109.4 | $ 111.4 |  |  |  |  |  |  |  |
MML and CML |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of investment |  |  |  | 3.60% |  |  |  |  |  |  |  |  |
Managed portfolio |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of investment |  |  |  | 5.10% | 8.20% |  |  |  |  |  |  |  |
Commercial mortgage loans |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Fair market value, trading |  |  |  | $ 163.6 | $ 156.6 |  |  |  |  |  |  |  |
Middle market loans |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Fair market value, trading |  |  |  | 112.1 | 111.7 |  |  |  |  |  |  |  |
Privately-held investments |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Fair market value, trading |  |  |  | 299.3 | 279.7 |  |  |  |  |  |  |  |
Bene |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Investment in the period |  | $ 3.3 |  |  |  |  |  |  |  |  |  |  |
Gross realized and unrealized gains (loss) |  |  |  | 0.0 | 0.0 | (0.9) |  |  |  |  |  |  |
Investment in Multiline reinsurer |  |  |  | 6.1 |  |  |  |  |  |  |  |  |
Equity method investment, ownership percentage |  | 20.00% |  |  |  |  |  |  |  |  |  |  |
Equity method investment, aggregate cost |  |  |  | 0.0 | 4.3 | 3.2 |  |  |  |  |  |  |
Equity method investment, cash payment |  |  |  | 0.0 | 1.1 | 1.2 |  |  |  |  |  |  |
Equity method investment, realized gain (loss) on disposal |  |  |  | 1.8 |  |  |  |  |  |  |  |  |
MVI |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Investment in the period |  |  | $ 0.8 | 0.0 | 0.0 | 0.2 | $ 0.1 |  |  |  |  |  |
Gross realized and unrealized gains (loss) |  |  |  | (0.4) | (0.1) | (0.2) |  |  |  |  |  |  |
Investment in Multiline reinsurer |  |  |  | 0.0 |  |  |  |  |  |  |  |  |
Equity method investment, aggregate cost |  |  |  | 0.0 | 0.4 | 0.5 |  |  |  |  |  |  |
Equity method investment, realized gain (loss) on disposal |  |  |  | 0.0 |  |  |  |  |  |  |  |  |
Digital Re |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Investment in the period |  |  |  | 0.0 | 0.0 |  |  |  |  |  |  |  |
Gross realized and unrealized gains (loss) |  |  |  | (0.3) | (0.2) | 0.4 |  |  |  |  |  |  |
Investment in Multiline reinsurer |  |  |  | 0.0 |  |  |  |  |  |  |  |  |
Equity method investment, ownership percentage |  |  |  |  |  |  |  |  |  |  |  |  | 49.00%
Equity method investment, aggregate cost |  |  |  | 0.4 | 0.7 | 0.9 |  |  |  |  |  |  | $ 2.3
Equity method investment, realized gain (loss) on disposal |  |  |  | 0.0 |  |  |  |  |  |  |  |  |
Crop Re |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Investment in the period |  |  |  | 0.0 | 0.0 |  |  |  |  |  |  |  |
Investment in Multiline reinsurer |  |  |  | 71.1 |  |  |  |  |  |  |  |  |
Equity method investment, ownership percentage | 23.20% |  |  |  |  |  |  |  |  |  |  | 23.20% |
Equity method investment, aggregate cost |  |  |  | 0.0 | 62.5 | 62.5 |  |  |  |  |  |  |
Equity method investment, realized gain (loss) on disposal | $ 71.1 |  |  | 8.6 |  |  |  |  |  |  |  |  |
Settlement for crop years | 14.5 |  |  |  |  |  |  |  |  |  |  |  |
Settlement re commutation agreement | $ 2.2 |  |  |  |  |  |  |  |  |  |  |  |
Multi-Line Insurer |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Investment in the period |  |  |  | 0.5 | 0.0 |  |  |  |  |  |  |  |
Investment in Multiline reinsurer |  |  |  | 0.0 |  |  |  |  |  |  |  |  |
Equity method investment, aggregate cost |  |  |  | 0.5 | 0.0 | 0.0 |  |  |  |  |  |  |
Equity method investment, realized gain (loss) on disposal |  |  |  | 0.0 |  |  |  |  |  |  |  |  |
Other investments |  |  |  | 0.3 |  |  |  | $ 0.2 | $ 5.0 |  |  |  |
Limited Partner | Real estate fund |  |  |  |  |  |  |  |  |  |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted assets |  |  |  |  |  | $ 100.0 |  |  |  |  |  |  |
Other investments |  |  |  | $ 109.4 | $ 111.4 |  |  |  |  | $ 13.8 | $ 86.2 |  |

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- Definition Amount of increase (decrease) in allowance for credit loss of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 30 -Section 50 -Paragraph 9 -URI http://asc.fasb.org/extlink&oid=118955579&loc=SL82922900-210455 + Details
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