Debt Securities (Gross Unrealized Losses on Investments in Debt Securities) (Detail) - USD ($) $ in Millions	Jun. 30, 2023	Dec. 31, 2022
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	$ 1,728.7	$ 3,865.7
Unrealized losses, Less than 12 months	(57.7)	(268.4)
Estimated fair value, 12 months or longer	5,261.6	3,780.1
Unrealized losses, 12 months or longer	(911.2)	(741.1)
Estimated fair value, Total	6,990.3	7,645.8
Unrealized losses, Total	(968.9)	(1,009.5)
U.S. Treasury bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	58.9	108.0
Unrealized losses, Less than 12 months	(0.6)	(1.4)
Estimated fair value, 12 months or longer	54.7	49.5
Unrealized losses, 12 months or longer	(5.9)	(5.7)
Estimated fair value, Total	113.6	157.5
Unrealized losses, Total	(6.5)	(7.1)
Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	315.3	813.4
Unrealized losses, Less than 12 months	(6.6)	(55.8)
Estimated fair value, 12 months or longer	822.1	540.0
Unrealized losses, 12 months or longer	(157.0)	(139.3)
Estimated fair value, Total	1,137.4	1,353.4
Unrealized losses, Total	(163.6)	(195.1)
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	88.6	142.1
Unrealized losses, Less than 12 months	(1.0)	(5.9)
Estimated fair value, 12 months or longer	88.0	45.7
Unrealized losses, 12 months or longer	(12.9)	(8.5)
Estimated fair value, Total	176.6	187.8
Unrealized losses, Total	(13.9)	(14.4)
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	113.7	172.7
Unrealized losses, Less than 12 months	(2.9)	(8.2)
Estimated fair value, 12 months or longer	110.7	55.8
Unrealized losses, 12 months or longer	(14.9)	(11.1)
Estimated fair value, Total	224.4	228.5
Unrealized losses, Total	(17.8)	(19.3)
Governmental agency mortgage-backed securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	884.3	1,859.6
Unrealized losses, Less than 12 months	(40.7)	(141.4)
Estimated fair value, 12 months or longer	3,304.5	2,626.8
Unrealized losses, 12 months or longer	(617.4)	(511.3)
Estimated fair value, Total	4,188.8	4,486.4
Unrealized losses, Total	(658.1)	(652.7)
U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	174.3	528.3
Unrealized losses, Less than 12 months	(3.6)	(38.2)
Estimated fair value, 12 months or longer	604.1	325.2
Unrealized losses, 12 months or longer	(73.3)	(46.3)
Estimated fair value, Total	778.4	853.5
Unrealized losses, Total	(76.9)	(84.5)
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Less than 12 months	93.6	241.6
Unrealized losses, Less than 12 months	(2.3)	(17.5)
Estimated fair value, 12 months or longer	277.5	137.1
Unrealized losses, 12 months or longer	(29.8)	(18.9)
Estimated fair value, Total	371.1	378.7
Unrealized losses, Total	$ (32.1)	$ (36.4)