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Debt Securities (Investments in Debt Securities, Classified as Available-For-Sale) (Detail) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	$ 7,774.6	$ 7,730.9
Debt Securities, Gross unrealized gains	31.2	19.6
Debt Securities, Gross unrealized losses	(371.8)	(484.6)
Debt securities, Estimated fair value	7,434.0	7,265.9
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	175.5	180.7
Debt Securities, Gross unrealized gains	0.5	0.1
Debt Securities, Gross unrealized losses	(3.1)	(5.2)
Debt securities, Estimated fair value	172.9	175.6
Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	864.2	844.9
Debt Securities, Gross unrealized gains	3.9	5.3
Debt Securities, Gross unrealized losses	(71.6)	(73.7)
Debt securities, Estimated fair value	796.5	776.5
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	215.8	217.1
Debt Securities, Gross unrealized gains	2.1	1.6
Debt Securities, Gross unrealized losses	(5.2)	(7.1)
Debt securities, Estimated fair value	212.7	211.6
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	202.2	203.8
Debt Securities, Gross unrealized gains	0.0	0.0
Debt Securities, Gross unrealized losses	(9.7)	(14.0)
Debt securities, Estimated fair value	192.5	189.8
Governmental agency mortgage-backed securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	4,848.2	4,844.4
Debt Securities, Gross unrealized gains	10.4	1.7
Debt Securities, Gross unrealized losses	(249.8)	(343.8)
Debt securities, Estimated fair value	4,608.8	4,502.3
U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	981.9	948.4
Debt Securities, Gross unrealized gains	7.7	5.6
Debt Securities, Gross unrealized losses	(23.1)	(28.4)
Debt securities, Estimated fair value	966.5	925.6
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	486.8	491.6
Debt Securities, Gross unrealized gains	6.6	5.3
Debt Securities, Gross unrealized losses	(9.3)	(12.4)
Debt securities, Estimated fair value	$ 484.1	$ 484.5

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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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