Debt Securities (Investments in Debt Securities, Classified as Available-For-Sale) (Detail) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	$ 8,478.2	$ 9,169.6
Debt Securities, Gross unrealized gains	16.3	9.5
Debt Securities, Gross unrealized losses	(869.8)	(1,009.5)
Debt securities, Estimated fair value	7,624.7	8,169.6
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	226.1	308.5
Debt Securities, Gross unrealized gains	0.5	1.4
Debt Securities, Gross unrealized losses	(5.4)	(7.1)
Debt securities, Estimated fair value	221.2	302.8
Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	1,397.1	1,670.6
Debt Securities, Gross unrealized gains	5.8	3.2
Debt Securities, Gross unrealized losses	(153.8)	(195.1)
Debt securities, Estimated fair value	1,249.1	1,478.7
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	206.6	208.0
Debt Securities, Gross unrealized gains	0.7	0.1
Debt Securities, Gross unrealized losses	(11.2)	(14.4)
Debt securities, Estimated fair value	196.1	193.7
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	246.8	247.9
Debt Securities, Gross unrealized gains	0.0	0.0
Debt Securities, Gross unrealized losses	(16.3)	(19.3)
Debt securities, Estimated fair value	230.5	228.6
Governmental agency mortgage-backed securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	5,023.0	5,253.4
Debt Securities, Gross unrealized gains	3.8	1.7
Debt Securities, Gross unrealized losses	(581.9)	(652.7)
Debt securities, Estimated fair value	4,444.9	4,602.4
U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	906.3	1,004.4
Debt Securities, Gross unrealized gains	3.0	1.5
Debt Securities, Gross unrealized losses	(71.0)	(84.5)
Debt securities, Estimated fair value	838.3	921.4
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	472.3	476.8
Debt Securities, Gross unrealized gains	2.5	1.6
Debt Securities, Gross unrealized losses	(30.2)	(36.4)
Debt securities, Estimated fair value	$ 444.6	$ 442.0

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).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
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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).

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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
-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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