Investments - Gross Unrealized losses by Major Security (Detail) $ in Millions	Jun. 30, 2024 USD ($) security	Dec. 31, 2023 USD ($) security	Jun. 30, 2023 USD ($) security
Number of Securities
Total | security	1,257	1,223	1,479
Less than 12 Months | security	301	126	306
12 Months or Greater | security	956	1,097	1,173
Fair Value
Total	$ 48,226.7	$ 44,062.1	$ 51,363.7
Less than 12 Months	18,659.5	13,347.6	22,029.5
12 Months or Greater	29,567.2	30,714.5	29,334.2
Unrealized Losses
Total	(2,267.4)	(2,346.8)	(3,367.6)
Less than 12 Months	(135.0)	(119.3)	(496.9)
12 Months or Greater	$ (2,132.4)	$ (2,227.5)	$ (2,870.7)
U.S. government obligations
Number of Securities
Total | security	117	147	171
Less than 12 Months | security	17	25	51
12 Months or Greater | security	100	122	120
Fair Value
Total	$ 29,776.3	$ 28,225.0	$ 31,496.1
Less than 12 Months	12,936.0	11,890.0	17,704.6
12 Months or Greater	16,840.3	16,335.0	13,791.5
Unrealized Losses
Total	(1,228.7)	(1,157.9)	(1,676.7)
Less than 12 Months	(109.4)	(100.0)	(427.9)
12 Months or Greater	$ (1,119.3)	$ (1,057.9)	$ (1,248.8)
State and local government obligations
Number of Securities
Total | security	336	324	346
Less than 12 Months | security	58	31	57
12 Months or Greater | security	278	293	289
Fair Value
Total	$ 2,078.5	$ 1,846.2	$ 1,998.2
Less than 12 Months	410.9	169.9	346.8
12 Months or Greater	1,667.6	1,676.3	1,651.4
Unrealized Losses
Total	(144.3)	(138.4)	(181.9)
Less than 12 Months	(2.0)	(0.9)	(5.2)
12 Months or Greater	$ (142.3)	$ (137.5)	$ (176.7)
Foreign government obligations
Number of Securities
Total | security	1	1	1
Less than 12 Months | security	0	0	0
12 Months or Greater | security	1	1	1
Fair Value
Total	$ 15.8	$ 16.3	$ 15.8
Less than 12 Months	0.0	0.0	0.0
12 Months or Greater	15.8	16.3	15.8
Unrealized Losses
Total	(0.9)	(1.0)	(1.4)
Less than 12 Months	0.0	0.0	0.0
12 Months or Greater	$ (0.9)	$ (1.0)	$ (1.4)
Corporate debt securities
Number of Securities
Total | security	402	313	414
Less than 12 Months | security	150	26	109
12 Months or Greater | security	252	287	305
Fair Value
Total	$ 8,995.0	$ 6,642.4	$ 8,565.9
Less than 12 Months	3,307.1	617.2	2,397.4
12 Months or Greater	5,687.9	6,025.2	6,168.5
Unrealized Losses
Total	(303.8)	(332.3)	(540.3)
Less than 12 Months	(18.4)	(14.7)	(56.0)
12 Months or Greater	$ (285.4)	$ (317.6)	$ (484.3)
Residential mortgage-backed securities
Number of Securities
Total | security	39	39	41
Less than 12 Months | security	7	2	7
12 Months or Greater | security	32	37	34
Fair Value
Total	$ 308.0	$ 88.4	$ 108.5
Less than 12 Months	244.1	0.4	2.4
12 Months or Greater	63.9	88.0	106.1
Unrealized Losses
Total	(9.4)	(10.0)	(13.0)
Less than 12 Months	(0.5)	0.0	(0.1)
12 Months or Greater	$ (8.9)	$ (10.0)	$ (12.9)
Commercial mortgage-backed securities
Number of Securities
Total | security	183	189	211
Less than 12 Months | security	7	1	4
12 Months or Greater | security	176	188	207
Fair Value
Total	$ 3,883.2	$ 3,912.2	$ 4,247.9
Less than 12 Months	314.4	30.7	13.6
12 Months or Greater	3,568.8	3,881.5	4,234.3
Unrealized Losses
Total	(489.1)	(597.7)	(718.7)
Less than 12 Months	(1.3)	(2.5)	(0.1)
12 Months or Greater	$ (487.8)	$ (595.2)	$ (718.6)
Other asset-backed securities
Number of Securities
Total | security	176	207	292
Less than 12 Months | security	62	41	78
12 Months or Greater | security	114	166	214
Fair Value
Total	$ 3,133.3	$ 3,299.1	$ 4,900.0
Less than 12 Months	1,447.0	639.4	1,564.7
12 Months or Greater	1,686.3	2,659.7	3,335.3
Unrealized Losses
Total	(89.3)	(107.1)	(232.0)
Less than 12 Months	(3.4)	(1.2)	(7.6)
12 Months or Greater	$ (85.9)	$ (105.9)	$ (224.4)
Redeemable preferred stocks
Number of Securities
Total | security	3	3	3
Less than 12 Months | security	0	0	0
12 Months or Greater | security	3	3	3
Fair Value
Total	$ 36.6	$ 32.5	$ 31.3
Less than 12 Months	0.0	0.0	0.0
12 Months or Greater	36.6	32.5	31.3
Unrealized Losses
Total	(1.9)	(2.4)	(3.6)
Less than 12 Months	0.0	0.0	0.0
12 Months or Greater	$ (1.9)	$ (2.4)	$ (3.6)