Investments - Gross Unrealized losses by Major Security (Detail) $ in Millions	Sep. 30, 2024 USD ($) security	Dec. 31, 2023 USD ($) security	Sep. 30, 2023 USD ($) security
Number of Securities
Total | security	903	1,223	1,531
Less than 12 Months | security	86	126	353
12 Months or Greater | security	817	1,097	1,178
Fair Value
Total	$ 25,490.0	$ 44,062.1	$ 54,926.2
Less than 12 Months	3,410.5	13,347.6	22,483.5
12 Months or Greater	22,079.5	30,714.5	32,442.7
Unrealized Losses
Total	(1,257.4)	(2,346.8)	(3,846.3)
Less than 12 Months	(5.6)	(119.3)	(668.1)
12 Months or Greater	$ (1,251.8)	$ (2,227.5)	$ (3,178.2)
U.S. government obligations
Number of Securities
Total | security	86	147	173
Less than 12 Months | security	3	25	37
12 Months or Greater | security	83	122	136
Fair Value
Total	$ 12,919.6	$ 28,225.0	$ 34,293.8
Less than 12 Months	2,069.3	11,890.0	16,686.2
12 Months or Greater	10,850.3	16,335.0	17,607.6
Unrealized Losses
Total	(574.2)	(1,157.9)	(2,098.4)
Less than 12 Months	(2.9)	(100.0)	(557.6)
12 Months or Greater	$ (571.3)	$ (1,057.9)	$ (1,540.8)
State and local government obligations
Number of Securities
Total | security	302	324	356
Less than 12 Months | security	44	31	46
12 Months or Greater | security	258	293	310
Fair Value
Total	$ 1,778.8	$ 1,846.2	$ 2,033.9
Less than 12 Months	207.8	169.9	359.6
12 Months or Greater	1,571.0	1,676.3	1,674.3
Unrealized Losses
Total	(90.3)	(138.4)	(203.3)
Less than 12 Months	(0.2)	(0.9)	(7.6)
12 Months or Greater	$ (90.1)	$ (137.5)	$ (195.7)
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	$ 16.4	$ 16.3	$ 15.4
Less than 12 Months	0.0	0.0	0.0
12 Months or Greater	16.4	16.3	15.4
Unrealized Losses
Total	(0.5)	(1.0)	(1.5)
Less than 12 Months	0.0	0.0	0.0
12 Months or Greater	$ (0.5)	$ (1.0)	$ (1.5)
Corporate and other debt securities
Number of Securities
Total | security	224	316	470
Less than 12 Months | security	14	26	164
12 Months or Greater | security	210	290	306
Fair Value
Total	$ 5,250.6	$ 6,674.9	$ 9,520.6
Less than 12 Months	426.1	617.2	3,380.4
12 Months or Greater	4,824.5	6,057.7	6,140.2
Unrealized Losses
Total	(155.3)	(334.7)	(596.1)
Less than 12 Months	(1.9)	(14.7)	(94.1)
12 Months or Greater	$ (153.4)	$ (320.0)	$ (502.0)
Residential mortgage-backed securities
Number of Securities
Total | security	33	39	40
Less than 12 Months | security	4	2	2
12 Months or Greater | security	29	37	38
Fair Value
Total	$ 244.2	$ 88.4	$ 99.9
Less than 12 Months	185.0	0.4	0.4
12 Months or Greater	59.2	88.0	99.5
Unrealized Losses
Total	(6.4)	(10.0)	(13.0)
Less than 12 Months	(0.2)	0.0	0.0
12 Months or Greater	$ (6.2)	$ (10.0)	$ (13.0)
Commercial mortgage-backed securities
Number of Securities
Total | security	177	189	196
Less than 12 Months | security	5	1	0
12 Months or Greater | security	172	188	196
Fair Value
Total	$ 3,749.9	$ 3,912.2	$ 3,960.8
Less than 12 Months	169.8	30.7	0.0
12 Months or Greater	3,580.1	3,881.5	3,960.8
Unrealized Losses
Total	(376.9)	(597.7)	(723.9)
Less than 12 Months	(0.3)	(2.5)	0.0
12 Months or Greater	$ (376.6)	$ (595.2)	$ (723.9)
Other asset-backed securities
Number of Securities
Total | security	80	207	295
Less than 12 Months | security	16	41	104
12 Months or Greater | security	64	166	191
Fair Value
Total	$ 1,530.5	$ 3,299.1	$ 5,001.8
Less than 12 Months	352.5	639.4	2,056.9
12 Months or Greater	1,178.0	2,659.7	2,944.9
Unrealized Losses
Total	(53.8)	(107.1)	(210.1)
Less than 12 Months	(0.1)	(1.2)	(8.8)
12 Months or Greater	$ (53.7)	$ (105.9)	$ (201.3)