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Investment Securities - Net Realized and Unrealized Investment Gains and Losses - (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items] |  |  |
Realized investment gains (losses) | $ 115.1 | $ 156.8 | $ 187.1
Unrealized investment gains (losses) | (313.4) | (395.2) | 529.7
Net realized and unrealized investment gains (losses) | (93.0) | (380.3) | 686.0
Gain (loss) on investments related to foreign currency exchange | (29.3) | (7.7) | 4.0
Gross realized investment gains | 129.9 | 212.3 | 214.4
Gross realized investment losses | 14.8 | 55.5 | 27.3
Investments Held |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Net realized and unrealized investment gains (losses) | (191.5) | (246.1) | 716.8
Fixed maturity investments |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Realized investment gains (losses) | (11.9) | 3.9 | 10.9
Unrealized investment gains (losses) | (168.4) | (42.2) | 27.5
Short-term investments |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Realized investment gains (losses) | (1.4) | (0.1) | 0.4
Unrealized investment gains (losses) | (1.1) | 0.0 | 0.0
Common equity securities |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Realized investment gains (losses) | 0.0 | 0.4 | 137.2
Unrealized investment gains (losses) | (7.1) | 14.9 | (130.6)
Net realized and unrealized investment gains (losses) | (7.1) | 15.3 | 6.6
MediaAlpha |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Realized investment gains (losses) | 0.0 | 160.3 | 63.8
Unrealized investment gains (losses) | (93.0) | (540.6) | 622.2
Net realized and unrealized investment gains (losses) | (93.0) | (380.3) | 686.0
Other long-term investments |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Realized investment gains (losses) | 128.4 | (7.7) | (25.2)
Unrealized investment gains (losses) | (43.8) | 172.7 | 10.6
Total Investments |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Net realized and unrealized investment gains (losses) | (198.3) | (238.4) | 716.8
Amount attributable to TPC Providers |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Net realized and unrealized investment gains (losses) | 6.8 | (7.7) | 0.0
Fixed maturity and short-term investments |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Net realized and unrealized investment gains (losses) | (182.8) | (38.4) | 38.8
Less: net realized and unrealized gains (losses) on investment securities sold during the period | (2.9) | (8.4) | (8.7)
Fixed maturity and short-term investments | Investments Held |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Net realized and unrealized investment gains (losses) | (179.9) | (30.0) | 47.5
Common equity securities and investment in MediaAlpha |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Net realized and unrealized investment gains (losses) | (100.1) | (365.0) | 692.6
Less: net realized and unrealized gains (losses) on investment securities sold during the period | 0.0 | 20.3 | 38.3
Common equity securities and investment in MediaAlpha | Investments Held |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Net realized and unrealized investment gains (losses) | $ (100.1) | $ (385.3) | $ 654.3

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- Definition Amount of realized gain on investment in debt security measured at fair value with change in fair value recognized in net income (trading). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (e) -URI https://asc.fasb.org/extlink&oid=126970911&loc=d3e27357-111563 + Details
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- Definition Amount of realized loss on investment in debt security measured at fair value with change in fair value recognized in net income (trading). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (e) -URI https://asc.fasb.org/extlink&oid=126970911&loc=d3e27357-111563 + Details
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- Definition Amount of realized and unrealized gain (loss) on investment. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (c) -URI https://asc.fasb.org/extlink&oid=126970911&loc=d3e27405-111563 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section S99 -Paragraph 2 -Subparagraph (SX 210.5-03.7(c),9(a)) -URI https://asc.fasb.org/extlink&oid=126953954&loc=SL114868664-224227 Reference 3: http://www.xbrl.org/2003/role/recommendedDisclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI https://asc.fasb.org/extlink&oid=126954810&loc=d3e3602-108585 + Details
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