Debt Securities (Gross Unrealized Losses on Investments in Debt Securities) (Detail) - USD ($) $ in Millions	Jun. 30, 2025	Dec. 31, 2024
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	$ 3,017.3	$ 4,076.1
Unrealized losses, Less than 12 months	(60.6)	(122.9)
Estimated fair value, 12 months or longer	1,972.6	2,098.9
Unrealized losses, 12 months or longer	(300.9)	(361.7)
Estimated fair value, Total	4,989.9	6,175.0
Unrealized losses, Total	(361.5)	(484.6)
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	9.4	81.8
Unrealized losses, Less than 12 months	(0.2)	(1.8)
Estimated fair value, 12 months or longer	41.8	52.9
Unrealized losses, 12 months or longer	(1.9)	(3.4)
Estimated fair value, Total	51.2	134.7
Unrealized losses, Total	(2.1)	(5.2)
Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	228.1	248.3
Unrealized losses, Less than 12 months	(9.5)	(4.4)
Estimated fair value, 12 months or longer	391.6	417.0
Unrealized losses, 12 months or longer	(65.0)	(69.3)
Estimated fair value, Total	619.7	665.3
Unrealized losses, Total	(74.5)	(73.7)
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	29.7	29.8
Unrealized losses, Less than 12 months	0.0	(0.2)
Estimated fair value, 12 months or longer	62.1	72.7
Unrealized losses, 12 months or longer	(5.8)	(6.9)
Estimated fair value, Total	91.8	102.5
Unrealized losses, Total	(5.8)	(7.1)
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	104.1	144.4
Unrealized losses, Less than 12 months	(0.4)	(5.3)
Estimated fair value, 12 months or longer	37.2	37.9
Unrealized losses, 12 months or longer	(7.6)	(8.7)
Estimated fair value, Total	141.3	182.3
Unrealized losses, Total	(8.0)	(14.0)
Governmental agency mortgage-backed securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	2,290.7	2,977.2
Unrealized losses, Less than 12 months	(47.0)	(98.4)
Estimated fair value, 12 months or longer	1,246.4	1,290.4
Unrealized losses, 12 months or longer	(198.4)	(245.4)
Estimated fair value, Total	3,537.1	4,267.6
Unrealized losses, Total	(245.4)	(343.8)
U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	258.6	435.2
Unrealized losses, Less than 12 months	(2.9)	(9.6)
Estimated fair value, 12 months or longer	105.6	117.5
Unrealized losses, 12 months or longer	(15.6)	(18.8)
Estimated fair value, Total	364.2	552.7
Unrealized losses, Total	(18.5)	(28.4)
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	96.7	159.4
Unrealized losses, Less than 12 months	(0.6)	(3.2)
Estimated fair value, 12 months or longer	87.9	110.5
Unrealized losses, 12 months or longer	(6.6)	(9.2)
Estimated fair value, Total	184.6	269.9
Unrealized losses, Total	$ (7.2)	$ (12.4)