Debt Securities (Investments in Debt Securities, Classified as Available-For-Sale) (Detail) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	$ 7,730.9	$ 7,895.2
Debt Securities, Gross unrealized gains	19.6	29.6
Debt Securities, Gross unrealized losses	(484.6)	(767.3)
Debt securities, Estimated fair value	7,265.9	7,157.5
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	180.7	203.3
Debt Securities, Gross unrealized gains	0.1	0.5
Debt Securities, Gross unrealized losses	(5.2)	(4.5)
Debt securities, Estimated fair value	175.6	199.3
Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	844.9	1,373.7
Debt Securities, Gross unrealized gains	5.3	8.8
Debt Securities, Gross unrealized losses	(73.7)	(136.7)
Debt securities, Estimated fair value	776.5	1,245.8
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	217.1	228.4
Debt Securities, Gross unrealized gains	1.6	1.4
Debt Securities, Gross unrealized losses	(7.1)	(10.5)
Debt securities, Estimated fair value	211.6	219.3
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	203.8	207.7
Debt Securities, Gross unrealized gains	0.0	0.2
Debt Securities, Gross unrealized losses	(14.0)	(12.5)
Debt securities, Estimated fair value	189.8	195.4
Governmental agency mortgage-backed securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	4,844.4	4,396.2
Debt Securities, Gross unrealized gains	1.7	6.3
Debt Securities, Gross unrealized losses	(343.8)	(526.8)
Debt securities, Estimated fair value	4,502.3	3,875.7
U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	948.4	1,007.0
Debt Securities, Gross unrealized gains	5.6	6.6
Debt Securities, Gross unrealized losses	(28.4)	(55.2)
Debt securities, Estimated fair value	925.6	958.4
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	491.6	478.9
Debt Securities, Gross unrealized gains	5.3	5.8
Debt Securities, Gross unrealized losses	(12.4)	(21.1)
Debt securities, Estimated fair value	$ 484.5	$ 463.6

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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