Investments - Gross Unrealized losses by Major Security (Detail) $ in Millions	Mar. 31, 2024 USD ($) security	Dec. 31, 2023 USD ($) security	Mar. 31, 2023 USD ($) security
Number of Securities
Total | security	1,245	1,223	1,416
Less than 12 Months | security	248	126	291
12 Months or Greater | security	997	1,097	1,125
Fair Value
Total	$ 50,113.1	$ 44,062.1	$ 38,603.0
Less than 12 Months	25,748.9	13,347.6	9,487.3
12 Months or Greater	24,364.2	30,714.5	29,115.7
Unrealized Losses
Total	(2,423.2)	(2,346.8)	(2,893.0)
Less than 12 Months	(321.6)	(119.3)	(219.9)
12 Months or Greater	$ (2,101.6)	$ (2,227.5)	$ (2,673.1)
U.S. government obligations
Number of Securities
Total | security	139	147	148
Less than 12 Months | security	37	25	40
12 Months or Greater | security	102	122	108
Fair Value
Total	$ 32,968.1	$ 28,225.0	$ 19,446.8
Less than 12 Months	21,634.6	11,890.0	5,938.5
12 Months or Greater	11,333.5	16,335.0	13,508.3
Unrealized Losses
Total	(1,327.3)	(1,157.9)	(1,209.6)
Less than 12 Months	(295.1)	(100.0)	(127.5)
12 Months or Greater	$ (1,032.2)	$ (1,057.9)	$ (1,082.1)
State and local government obligations
Number of Securities
Total | security	330	324	343
Less than 12 Months | security	41	31	72
12 Months or Greater | security	289	293	271
Fair Value
Total	$ 1,922.7	$ 1,846.2	$ 1,943.6
Less than 12 Months	250.7	169.9	350.7
12 Months or Greater	1,672.0	1,676.3	1,592.9
Unrealized Losses
Total	(142.4)	(138.4)	(161.7)
Less than 12 Months	(1.8)	(0.9)	(7.4)
12 Months or Greater	$ (140.6)	$ (137.5)	$ (154.3)
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	$ 15.9	$ 16.3	$ 15.8
Less than 12 Months	0.0	0.0	0.0
12 Months or Greater	15.9	16.3	15.8
Unrealized Losses
Total	(1.0)	(1.0)	(1.1)
Less than 12 Months	0.0	0.0	0.0
12 Months or Greater	$ (1.0)	$ (1.0)	$ (1.1)
Corporate debt securities
Number of Securities
Total | security	363	313	390
Less than 12 Months | security	106	26	93
12 Months or Greater | security	257	287	297
Fair Value
Total	$ 8,094.3	$ 6,642.4	$ 8,129.9
Less than 12 Months	2,369.6	617.2	2,239.3
12 Months or Greater	5,724.7	6,025.2	5,890.6
Unrealized Losses
Total	(328.1)	(332.3)	(525.4)
Less than 12 Months	(20.6)	(14.7)	(75.5)
12 Months or Greater	$ (307.5)	$ (317.6)	$ (449.9)
Residential mortgage-backed securities
Number of Securities
Total | security	35	39	43
Less than 12 Months | security	1	2	13
12 Months or Greater | security	34	37	30
Fair Value
Total	$ 78.0	$ 88.4	$ 141.7
Less than 12 Months	0.0	0.4	10.0
12 Months or Greater	78.0	88.0	131.7
Unrealized Losses
Total	(9.2)	(10.0)	(16.3)
Less than 12 Months	0.0	0.0	(0.4)
12 Months or Greater	$ (9.2)	$ (10.0)	$ (15.9)
Commercial mortgage-backed securities
Number of Securities
Total | security	181	189	220
Less than 12 Months | security	4	1	11
12 Months or Greater | security	177	188	209
Fair Value
Total	$ 3,741.7	$ 3,912.2	$ 4,484.5
Less than 12 Months	190.5	30.7	53.4
12 Months or Greater	3,551.2	3,881.5	4,431.1
Unrealized Losses
Total	(519.3)	(597.7)	(751.2)
Less than 12 Months	(1.8)	(2.5)	(1.5)
12 Months or Greater	$ (517.5)	$ (595.2)	$ (749.7)
Other asset-backed securities
Number of Securities
Total | security	193	207	267
Less than 12 Months | security	59	41	61
12 Months or Greater | security	134	166	206
Fair Value
Total	$ 3,259.5	$ 3,299.1	$ 4,391.9
Less than 12 Months	1,303.5	639.4	884.8
12 Months or Greater	1,956.0	2,659.7	3,507.1
Unrealized Losses
Total	(93.9)	(107.1)	(222.5)
Less than 12 Months	(2.3)	(1.2)	(6.2)
12 Months or Greater	$ (91.6)	$ (105.9)	$ (216.3)
Redeemable preferred stocks
Number of Securities
Total | security	3	3	4
Less than 12 Months | security	0	0	1
12 Months or Greater | security	3	3	3
Fair Value
Total	$ 32.9	$ 32.5	$ 48.8
Less than 12 Months	0.0	0.0	10.6
12 Months or Greater	32.9	32.5	38.2
Unrealized Losses
Total	(2.0)	(2.4)	(5.2)
Less than 12 Months	0.0	0.0	(1.4)
12 Months or Greater	$ (2.0)	$ (2.4)	$ (3.8)