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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, 2021 | Dec. 31, 2020 | Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items] |  |  |
Realized investment gains (losses) | $ 156.8 | $ 187.1 | $ 96.6
Unrealized investment gains (losses) | (395.2) | 529.7 | 336.6
Gain (loss) on investments related to foreign currency exchange | (7.7) | 4.0 | (0.3)
Gross realized investment gains | 212.3 | 214.4 | 104.0
Gross realized investment losses | 55.5 | 27.3 | 7.4
Investments Held |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Net realized and unrealized investment gains (losses) | (246.1) | 716.8 | 433.2
Fixed maturity investments |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Realized investment gains (losses) | 3.9 | 10.9 | 4.9
Unrealized investment gains (losses) | (42.2) | 27.5 | 38.6
Short-term investments |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Realized investment gains (losses) | (0.1) | 0.4 | 0.2
Common equity securities |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Realized investment gains (losses) | 0.4 | 137.2 | 85.9
Unrealized investment gains (losses) | 14.9 | (130.6) | 109.8
Net realized and unrealized investment gains (losses) | 15.3 | 6.6 | 195.7
MediaAlpha |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Realized investment gains (losses) | 160.3 | 63.8 | 0.0
Unrealized investment gains (losses) | (540.6) | 622.2 | 180.0
Net realized and unrealized investment gains (losses) | (380.3) | 686.0 | 180.0
Other long-term investments |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Realized investment gains (losses) | (7.7) | (25.2) | 5.6
Unrealized investment gains (losses) | 172.7 | 10.6 | 8.2
Total Investments |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Net realized and unrealized investment gains (losses) | (238.4) | 716.8 | 433.2
Amount attributable to TPC Providers |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Net realized and unrealized investment gains (losses) | (7.7) | 0.0 | 0.0
Fixed maturity and short-term investments |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Net realized and unrealized investment gains (losses) | (38.4) | 38.8 | 43.7
Less: net realized and unrealized gains (losses) on investment securities sold during the period | (8.4) | (8.7) | 1.0
Fixed maturity and short-term investments | Investments Held |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Net realized and unrealized investment gains (losses) | (30.0) | 47.5 | 42.7
Common equity securities and investment in MediaAlpha |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Net realized and unrealized investment gains (losses) | (365.0) | 692.6 | 375.7
Less: net realized and unrealized gains (losses) on investment securities sold during the period | 20.3 | 38.3 | 24.0
Common equity securities and investment in MediaAlpha | Investments Held |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |
Net realized and unrealized investment gains (losses) | $ (385.3) | $ 654.3 | $ 351.7

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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 http://asc.fasb.org/extlink&oid=123581744&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 http://asc.fasb.org/extlink&oid=123581744&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 http://asc.fasb.org/extlink&oid=123581744&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 http://asc.fasb.org/extlink&oid=123367319&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 http://asc.fasb.org/extlink&oid=123570139&loc=d3e3602-108585 + Details
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