Debt Securities (Gross Unrealized Losses on Investments in Debt Securities) (Detail) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	$ 3,584.7	$ 4,076.1
Unrealized losses, Less than 12 months	(67.6)	(122.9)
Estimated fair value, 12 months or longer	2,059.0	2,098.9
Unrealized losses, 12 months or longer	(304.2)	(361.7)
Estimated fair value, Total	5,643.7	6,175.0
Unrealized losses, Total	(371.8)	(484.6)
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	38.4	81.8
Unrealized losses, Less than 12 months	(0.7)	(1.8)
Estimated fair value, 12 months or longer	45.8	52.9
Unrealized losses, 12 months or longer	(2.4)	(3.4)
Estimated fair value, Total	84.2	134.7
Unrealized losses, Total	(3.1)	(5.2)
Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	268.0	248.3
Unrealized losses, Less than 12 months	(5.8)	(4.4)
Estimated fair value, 12 months or longer	412.0	417.0
Unrealized losses, 12 months or longer	(65.8)	(69.3)
Estimated fair value, Total	680.0	665.3
Unrealized losses, Total	(71.6)	(73.7)
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	4.4	29.8
Unrealized losses, Less than 12 months	0.0	(0.2)
Estimated fair value, 12 months or longer	63.7	72.7
Unrealized losses, 12 months or longer	(5.2)	(6.9)
Estimated fair value, Total	68.1	102.5
Unrealized losses, Total	(5.2)	(7.1)
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	146.3	144.4
Unrealized losses, Less than 12 months	(1.8)	(5.3)
Estimated fair value, 12 months or longer	37.8	37.9
Unrealized losses, 12 months or longer	(7.9)	(8.7)
Estimated fair value, Total	184.1	182.3
Unrealized losses, Total	(9.7)	(14.0)
Governmental agency mortgage-backed securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	2,558.4	2,977.2
Unrealized losses, Less than 12 months	(52.1)	(98.4)
Estimated fair value, 12 months or longer	1,283.8	1,290.4
Unrealized losses, 12 months or longer	(197.7)	(245.4)
Estimated fair value, Total	3,842.2	4,267.6
Unrealized losses, Total	(249.8)	(343.8)
U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	415.7	435.2
Unrealized losses, Less than 12 months	(5.5)	(9.6)
Estimated fair value, 12 months or longer	118.1	117.5
Unrealized losses, 12 months or longer	(17.6)	(18.8)
Estimated fair value, Total	533.8	552.7
Unrealized losses, Total	(23.1)	(28.4)
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	153.5	159.4
Unrealized losses, Less than 12 months	(1.7)	(3.2)
Estimated fair value, 12 months or longer	97.8	110.5
Unrealized losses, 12 months or longer	(7.6)	(9.2)
Estimated fair value, Total	251.3	269.9
Unrealized losses, Total	$ (9.3)	$ (12.4)