Debt Securities (Gross Unrealized Losses on Investments in Debt Securities) (Detail) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	$ 4,076.1	$ 552.0
Unrealized losses, Less than 12 months	(122.9)	(8.2)
Estimated fair value, 12 months or longer	2,098.9	5,327.6
Unrealized losses, 12 months or longer	(361.7)	(759.1)
Estimated fair value, Total	6,175.0	5,879.6
Unrealized losses, Total	(484.6)	(767.3)
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	81.8	8.2
Unrealized losses, Less than 12 months	(1.8)	(0.1)
Estimated fair value, 12 months or longer	52.9	55.4
Unrealized losses, 12 months or longer	(3.4)	(4.4)
Estimated fair value, Total	134.7	63.6
Unrealized losses, Total	(5.2)	(4.5)
Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	248.3	107.4
Unrealized losses, Less than 12 months	(4.4)	(0.9)
Estimated fair value, 12 months or longer	417.0	956.8
Unrealized losses, 12 months or longer	(69.3)	(135.8)
Estimated fair value, Total	665.3	1,064.2
Unrealized losses, Total	(73.7)	(136.7)
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	29.8	33.3
Unrealized losses, Less than 12 months	(0.2)	(0.1)
Estimated fair value, 12 months or longer	72.7	101.4
Unrealized losses, 12 months or longer	(6.9)	(10.4)
Estimated fair value, Total	102.5	134.7
Unrealized losses, Total	(7.1)	(10.5)
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	144.4	0.4
Unrealized losses, Less than 12 months	(5.3)	0.0
Estimated fair value, 12 months or longer	37.9	118.9
Unrealized losses, 12 months or longer	(8.7)	(12.5)
Estimated fair value, Total	182.3	119.3
Unrealized losses, Total	(14.0)	(12.5)
Governmental agency mortgage-backed securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	2,977.2	338.3
Unrealized losses, Less than 12 months	(98.4)	(6.6)
Estimated fair value, 12 months or longer	1,290.4	3,225.3
Unrealized losses, 12 months or longer	(245.4)	(520.2)
Estimated fair value, Total	4,267.6	3,563.6
Unrealized losses, Total	(343.8)	(526.8)
U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	435.2	45.1
Unrealized losses, Less than 12 months	(9.6)	(0.4)
Estimated fair value, 12 months or longer	117.5	602.5
Unrealized losses, 12 months or longer	(18.8)	(54.8)
Estimated fair value, Total	552.7	647.6
Unrealized losses, Total	(28.4)	(55.2)
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	159.4	19.3
Unrealized losses, Less than 12 months	(3.2)	(0.1)
Estimated fair value, 12 months or longer	110.5	267.3
Unrealized losses, 12 months or longer	(9.2)	(21.0)
Estimated fair value, Total	269.9	286.6
Unrealized losses, Total	$ (12.4)	$ (21.1)

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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