Debt Securities (Gross Unrealized Losses on Investments in Debt Securities) (Detail) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	$ 1,843.8	$ 552.0
Unrealized losses, Less than 12 months	(11.5)	(8.2)
Estimated fair value, 12 months or longer	2,332.7	5,327.6
Unrealized losses, 12 months or longer	(291.8)	(759.1)
Estimated fair value, Total	4,176.5	5,879.6
Unrealized losses, Total	(303.3)	(767.3)
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	1.3	8.2
Unrealized losses, Less than 12 months	0.0	(0.1)
Estimated fair value, 12 months or longer	57.9	55.4
Unrealized losses, 12 months or longer	(3.0)	(4.4)
Estimated fair value, Total	59.2	63.6
Unrealized losses, Total	(3.0)	(4.5)
Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	49.7	107.4
Unrealized losses, Less than 12 months	(0.2)	(0.9)
Estimated fair value, 12 months or longer	455.5	956.8
Unrealized losses, 12 months or longer	(55.1)	(135.8)
Estimated fair value, Total	505.2	1,064.2
Unrealized losses, Total	(55.3)	(136.7)
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	10.4	33.3
Unrealized losses, Less than 12 months	0.0	(0.1)
Estimated fair value, 12 months or longer	78.2	101.4
Unrealized losses, 12 months or longer	(6.6)	(10.4)
Estimated fair value, Total	88.6	134.7
Unrealized losses, Total	(6.6)	(10.5)
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	172.8	0.4
Unrealized losses, Less than 12 months	(0.4)	0.0
Estimated fair value, 12 months or longer	40.4	118.9
Unrealized losses, 12 months or longer	(7.3)	(12.5)
Estimated fair value, Total	213.2	119.3
Unrealized losses, Total	(7.7)	(12.5)
Governmental agency mortgage-backed securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	1,351.7	338.3
Unrealized losses, Less than 12 months	(9.7)	(6.6)
Estimated fair value, 12 months or longer	1,419.2	3,225.3
Unrealized losses, 12 months or longer	(196.2)	(520.2)
Estimated fair value, Total	2,770.9	3,563.6
Unrealized losses, Total	(205.9)	(526.8)
U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	189.5	45.1
Unrealized losses, Less than 12 months	(0.7)	(0.4)
Estimated fair value, 12 months or longer	140.9	602.5
Unrealized losses, 12 months or longer	(15.4)	(54.8)
Estimated fair value, Total	330.4	647.6
Unrealized losses, Total	(16.1)	(55.2)
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	68.4	19.3
Unrealized losses, Less than 12 months	(0.5)	(0.1)
Estimated fair value, 12 months or longer	140.6	267.3
Unrealized losses, 12 months or longer	(8.2)	(21.0)
Estimated fair value, Total	209.0	286.6
Unrealized losses, Total	$ (8.7)	$ (21.1)