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Debt Securities (Investments in Debt Securities, Classified as Available-For-Sale) (Detail) - USD ($) $ in Millions	Jun. 30, 2025	Dec. 31, 2024
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	$ 8,334.2	$ 7,730.9
Debt Securities, Gross unrealized gains	46.2	19.6
Debt Securities, Gross unrealized losses	(361.5)	(484.6)
Debt securities, Estimated fair value	8,018.9	7,265.9
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	171.2	180.7
Debt Securities, Gross unrealized gains	1.1	0.1
Debt Securities, Gross unrealized losses	(2.1)	(5.2)
Debt securities, Estimated fair value	170.2	175.6
Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	946.6	844.9
Debt Securities, Gross unrealized gains	4.6	5.3
Debt Securities, Gross unrealized losses	(74.5)	(73.7)
Debt securities, Estimated fair value	876.7	776.5
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	229.8	217.1
Debt Securities, Gross unrealized gains	1.5	1.6
Debt Securities, Gross unrealized losses	(5.8)	(7.1)
Debt securities, Estimated fair value	225.5	211.6
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	214.1	203.8
Debt Securities, Gross unrealized gains	0.2	0.0
Debt Securities, Gross unrealized losses	(8.0)	(14.0)
Debt securities, Estimated fair value	206.3	189.8
Governmental agency mortgage-backed securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	5,265.4	4,844.4
Debt Securities, Gross unrealized gains	17.9	1.7
Debt Securities, Gross unrealized losses	(245.4)	(343.8)
Debt securities, Estimated fair value	5,037.9	4,502.3
U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	1,016.2	948.4
Debt Securities, Gross unrealized gains	12.3	5.6
Debt Securities, Gross unrealized losses	(18.5)	(28.4)
Debt securities, Estimated fair value	1,010.0	925.6
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	490.9	491.6
Debt Securities, Gross unrealized gains	8.6	5.3
Debt Securities, Gross unrealized losses	(7.2)	(12.4)
Debt securities, Estimated fair value	$ 492.3	$ 484.5

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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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