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Debt Securities (Investments in Debt Securities, Classified as Available-For-Sale) (Detail) - USD ($) $ in Millions | Mar. 31, 2024 | Dec. 31, 2023
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Schedule Of Available For Sale Securities [Line Items] |  |
Debt Securities, Amortized cost | $ 7,859.0 | $ 7,895.2
Debt Securities, Gross unrealized gains | 17.9 | 29.6
Debt Securities, Gross unrealized losses | (822.5) | (767.3)
Debt securities, Estimated fair value | 7,054.4 | 7,157.5
U.S. Treasury bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Debt Securities, Amortized cost | 163.1 | 203.3
Debt Securities, Gross unrealized gains | 0.0 | 0.5
Debt Securities, Gross unrealized losses | (5.9) | (4.5)
Debt securities, Estimated fair value | 157.2 | 199.3
Municipal bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Debt Securities, Amortized cost | 1,381.2 | 1,373.7
Debt Securities, Gross unrealized gains | 7.1 | 8.8
Debt Securities, Gross unrealized losses | (139.0) | (136.7)
Debt securities, Estimated fair value | 1,249.3 | 1,245.8
Foreign government bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Debt Securities, Amortized cost | 209.9 | 228.4
Debt Securities, Gross unrealized gains | 0.2 | 1.4
Debt Securities, Gross unrealized losses | (10.7) | (10.5)
Debt securities, Estimated fair value | 199.4 | 219.3
Governmental agency bonds |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Debt Securities, Amortized cost | 214.3 | 207.7
Debt Securities, Gross unrealized gains | 0.0 | 0.2
Debt Securities, Gross unrealized losses | (13.9) | (12.5)
Debt securities, Estimated fair value | 200.4 | 195.4
Governmental agency mortgage-backed securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Debt Securities, Amortized cost | 4,316.7 | 4,396.2
Debt Securities, Gross unrealized gains | 2.2 | 6.3
Debt Securities, Gross unrealized losses | (573.0) | (526.8)
Debt securities, Estimated fair value | 3,745.9 | 3,875.7
U.S. corporate debt securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Debt Securities, Amortized cost | 1,075.4 | 1,007.0
Debt Securities, Gross unrealized gains | 4.4 | 6.6
Debt Securities, Gross unrealized losses | (58.6) | (55.2)
Debt securities, Estimated fair value | 1,021.2 | 958.4
Foreign corporate debt securities |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Debt Securities, Amortized cost | 498.4 | 478.9
Debt Securities, Gross unrealized gains | 4.0 | 5.8
Debt Securities, Gross unrealized losses | (21.4) | (21.1)
Debt securities, Estimated fair value | $ 481.0 | $ 463.6

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- Definition Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on 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/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-2 + Details
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- Definition Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on 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/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-2 + Details
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- Definition Amortized cost 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/2003/role/disclosureRef -Topic 944 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(1)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479440/944-210-S99-1 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481800/320-10-50-2 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 30 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org//1943274/2147479130/326-30-45-1 + Details
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