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Debt Securities (Investments in Debt Securities, Classified as Available-For-Sale) (Detail) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	$ 8,469.8	$ 7,730.9
Debt Securities, Gross unrealized gains	70.7	19.6
Debt Securities, Gross unrealized losses	(292.3)	(484.6)
Debt securities, Estimated fair value	8,248.2	7,265.9
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	249.4	180.7
Debt Securities, Gross unrealized gains	1.4	0.1
Debt Securities, Gross unrealized losses	(1.9)	(5.2)
Debt securities, Estimated fair value	248.9	175.6
Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	961.2	844.9
Debt Securities, Gross unrealized gains	8.8	5.3
Debt Securities, Gross unrealized losses	(59.7)	(73.7)
Debt securities, Estimated fair value	910.3	776.5
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	226.5	217.1
Debt Securities, Gross unrealized gains	1.8	1.6
Debt Securities, Gross unrealized losses	(4.7)	(7.1)
Debt securities, Estimated fair value	223.6	211.6
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	243.0	203.8
Debt Securities, Gross unrealized gains	0.3	0.0
Debt Securities, Gross unrealized losses	(7.6)	(14.0)
Debt securities, Estimated fair value	235.7	189.8
Governmental agency mortgage-backed securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	5,257.2	4,844.4
Debt Securities, Gross unrealized gains	31.6	1.7
Debt Securities, Gross unrealized losses	(198.2)	(343.8)
Debt securities, Estimated fair value	5,090.6	4,502.3
U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	1,048.8	948.4
Debt Securities, Gross unrealized gains	16.4	5.6
Debt Securities, Gross unrealized losses	(15.0)	(28.4)
Debt securities, Estimated fair value	1,050.2	925.6
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	483.7	491.6
Debt Securities, Gross unrealized gains	10.4	5.3
Debt Securities, Gross unrealized losses	(5.2)	(12.4)
Debt securities, Estimated fair value	$ 488.9	$ 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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