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Debt Securities (Investments in Debt Securities, Classified as Available-For-Sale) (Detail) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	$ 7,256.7	$ 7,895.2
Debt Securities, Gross unrealized gains	61.9	29.6
Debt Securities, Gross unrealized losses	(303.3)	(767.3)
Debt securities, Estimated fair value	7,015.3	7,157.5
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	191.8	203.3
Debt Securities, Gross unrealized gains	1.6	0.5
Debt Securities, Gross unrealized losses	(3.0)	(4.5)
Debt securities, Estimated fair value	190.4	199.3
Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	777.2	1,373.7
Debt Securities, Gross unrealized gains	9.5	8.8
Debt Securities, Gross unrealized losses	(55.3)	(136.7)
Debt securities, Estimated fair value	731.4	1,245.8
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	230.7	228.4
Debt Securities, Gross unrealized gains	2.5	1.4
Debt Securities, Gross unrealized losses	(6.6)	(10.5)
Debt securities, Estimated fair value	226.6	219.3
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	229.6	207.7
Debt Securities, Gross unrealized gains	0.1	0.2
Debt Securities, Gross unrealized losses	(7.7)	(12.5)
Debt securities, Estimated fair value	222.0	195.4
Governmental agency mortgage-backed securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	4,468.8	4,396.2
Debt Securities, Gross unrealized gains	23.0	6.3
Debt Securities, Gross unrealized losses	(205.9)	(526.8)
Debt securities, Estimated fair value	4,285.9	3,875.7
U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	865.2	1,007.0
Debt Securities, Gross unrealized gains	15.4	6.6
Debt Securities, Gross unrealized losses	(16.1)	(55.2)
Debt securities, Estimated fair value	864.5	958.4
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	493.4	478.9
Debt Securities, Gross unrealized gains	9.8	5.8
Debt Securities, Gross unrealized losses	(8.7)	(21.1)
Debt securities, Estimated fair value	$ 494.5	$ 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).

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