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Debt Securities (Investments in Debt Securities, Classified as Available-For-Sale) (Detail) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	$ 8,414.7	$ 9,169.6
Debt Securities, Gross unrealized gains	2.4	9.5
Debt Securities, Gross unrealized losses	(1,189.9)	(1,009.5)
Debt securities, Estimated fair value	7,227.2	8,169.6
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	136.1	308.5
Debt Securities, Gross unrealized gains	0.0	1.4
Debt Securities, Gross unrealized losses	(8.0)	(7.1)
Debt securities, Estimated fair value	128.1	302.8
Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	1,403.0	1,670.6
Debt Securities, Gross unrealized gains	0.9	3.2
Debt Securities, Gross unrealized losses	(207.8)	(195.1)
Debt securities, Estimated fair value	1,196.1	1,478.7
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	224.0	208.0
Debt Securities, Gross unrealized gains	0.0	0.1
Debt Securities, Gross unrealized losses	(17.1)	(14.4)
Debt securities, Estimated fair value	206.9	193.7
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	237.7	247.9
Debt Securities, Gross unrealized gains	0.0	0.0
Debt Securities, Gross unrealized losses	(20.2)	(19.3)
Debt securities, Estimated fair value	217.5	228.6
Governmental agency mortgage-backed securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	4,987.1	5,253.4
Debt Securities, Gross unrealized gains	0.1	1.7
Debt Securities, Gross unrealized losses	(816.8)	(652.7)
Debt securities, Estimated fair value	4,170.4	4,602.4
U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	965.0	1,004.4
Debt Securities, Gross unrealized gains	0.6	1.5
Debt Securities, Gross unrealized losses	(85.5)	(84.5)
Debt securities, Estimated fair value	880.1	921.4
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	461.8	476.8
Debt Securities, Gross unrealized gains	0.8	1.6
Debt Securities, Gross unrealized losses	(34.5)	(36.4)
Debt securities, Estimated fair value	$ 428.1	$ 442.0

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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

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