Debt Securities (Investments in Debt Securities, Classified as Available-For-Sale) (Detail) - USD ($) $ in Millions	Jun. 30, 2026	Dec. 31, 2025
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	$ 10,062.0	$ 8,656.7
Debt Securities, Gross unrealized gains	38.9	79.4
Debt Securities, Gross unrealized losses	(326.2)	(269.4)
Debt securities, Estimated fair value	9,774.7	8,466.7
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	221.9	244.2
Debt Securities, Gross unrealized gains	0.0	1.0
Debt Securities, Gross unrealized losses	(3.2)	(1.6)
Debt securities, Estimated fair value	218.7	243.6
Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	1,099.9	1,003.5
Debt Securities, Gross unrealized gains	8.3	9.9
Debt Securities, Gross unrealized losses	(54.9)	(53.3)
Debt securities, Estimated fair value	1,053.3	960.1
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	235.6	239.4
Debt Securities, Gross unrealized gains	1.1	1.1
Debt Securities, Gross unrealized losses	(3.4)	(5.3)
Debt securities, Estimated fair value	233.3	235.2
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	383.9	268.3
Debt Securities, Gross unrealized gains	0.1	0.3
Debt Securities, Gross unrealized losses	(9.4)	(7.6)
Debt securities, Estimated fair value	374.6	261.0
Governmental agency mortgage-backed securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	6,339.9	5,401.9
Debt Securities, Gross unrealized gains	18.0	41.9
Debt Securities, Gross unrealized losses	(227.3)	(183.0)
Debt securities, Estimated fair value	6,130.6	5,260.8
U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	1,284.9	1,032.8
Debt Securities, Gross unrealized gains	6.4	15.7
Debt Securities, Gross unrealized losses	(21.4)	(13.7)
Debt securities, Estimated fair value	1,269.9	1,034.8
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	495.9	466.6
Debt Securities, Gross unrealized gains	5.0	9.5
Debt Securities, Gross unrealized losses	(6.6)	(4.9)
Debt securities, Estimated fair value	$ 494.3	$ 471.2

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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