Investments - Gross Unrealized losses by Major Security (Detail) $ in Millions	Jun. 30, 2022 USD ($) security	Dec. 31, 2021 USD ($) security	Jun. 30, 2021 USD ($) security
Number of Securities
Total | security	1,525	806	697
Less than 12 Months | security	1,407	781	653
12 Months or Greater | security	118	25	44
Fair Value
Total	$ 40,278.0	$ 26,867.3	$ 14,757.0
Less than 12 Months	36,394.7	25,840.8	13,850.6
12 Months or Greater	3,883.3	1,026.5	906.4
Unrealized Losses
Total	(2,784.1)	(316.0)	(101.6)
Less than 12 Months	(2,384.7)	(281.6)	(91.4)
12 Months or Greater	$ (399.4)	$ (34.4)	$ (10.2)
U.S. government obligations
Number of Securities
Total | security	140	92	59
Less than 12 Months | security	114	85	55
12 Months or Greater | security	26	7	4
Fair Value
Total	$ 17,907.2	$ 14,745.8	$ 10,629.4
Less than 12 Months	15,254.3	13,790.8	10,162.4
12 Months or Greater	2,652.9	955.0	467.0
Unrealized Losses
Total	(1,116.8)	(190.8)	(72.9)
Less than 12 Months	(874.0)	(158.5)	(66.8)
12 Months or Greater	$ (242.8)	$ (32.3)	$ (6.1)
State and local government obligations
Number of Securities
Total | security	331	127	81
Less than 12 Months | security	318	122	72
12 Months or Greater | security	13	5	9
Fair Value
Total	$ 1,954.9	$ 954.2	$ 436.2
Less than 12 Months	1,816.1	927.3	400.0
12 Months or Greater	138.8	26.9	36.2
Unrealized Losses
Total	(147.1)	(14.0)	(3.3)
Less than 12 Months	(126.1)	(13.1)	(2.6)
12 Months or Greater	$ (21.0)	$ (0.9)	$ (0.7)
Foreign government obligations
Number of Securities
Total | security	1
Less than 12 Months | security	1
12 Months or Greater | security	0
Fair Value
Total	$ 16.4
Less than 12 Months	16.4
12 Months or Greater	0.0
Unrealized Losses
Total	(1.3)
Less than 12 Months	(1.3)
12 Months or Greater	$ 0.0
Corporate debt securities
Number of Securities
Total | security	466	220	292
Less than 12 Months | security	435	219	288
12 Months or Greater | security	31	1	4
Fair Value
Total	$ 9,174.2	$ 3,496.6	$ 1,142.5
Less than 12 Months	8,843.2	3,491.7	1,065.2
12 Months or Greater	331.0	4.9	77.3
Unrealized Losses
Total	(655.0)	(33.4)	(9.9)
Less than 12 Months	(612.0)	(33.3)	(9.5)
12 Months or Greater	$ (43.0)	$ (0.1)	$ (0.4)
Residential mortgage-backed securities
Number of Securities
Total | security	39	20	77
Less than 12 Months | security	32	14	64
12 Months or Greater | security	7	6	13
Fair Value
Total	$ 183.7	$ 138.6	$ 192.2
Less than 12 Months	172.1	135.4	172.4
12 Months or Greater	11.6	3.2	19.8
Unrealized Losses
Total	(10.7)	(0.6)	(1.1)
Less than 12 Months	(8.8)	(0.5)	(0.7)
12 Months or Greater	$ (1.9)	$ (0.1)	$ (0.4)
Commercial mortgage-backed securities
Number of Securities
Total | security	254	168	68
Less than 12 Months | security	234	165	61
12 Months or Greater | security	20	3	7
Fair Value
Total	$ 6,058.4	$ 4,315.4	$ 1,272.0
Less than 12 Months	5,624.1	4,295.0	1,006.9
12 Months or Greater	434.3	20.4	265.1
Unrealized Losses
Total	(646.0)	(64.3)	(11.5)
Less than 12 Months	(569.8)	(63.9)	(10.5)
12 Months or Greater	$ (76.2)	$ (0.4)	$ (1.0)
Other asset-backed securities
Number of Securities
Total | security	289	178	119
Less than 12 Months | security	269	176	113
12 Months or Greater | security	20	2	6
Fair Value
Total	$ 4,915.7	$ 3,204.7	$ 1,073.6
Less than 12 Months	4,611.8	3,200.6	1,043.7
12 Months or Greater	303.9	4.1	29.9
Unrealized Losses
Total	(201.7)	(12.4)	(1.5)
Less than 12 Months	(188.9)	(12.3)	(1.3)
12 Months or Greater	$ (12.8)	$ (0.1)	$ (0.2)
Redeemable preferred stocks
Number of Securities
Total | security	5	1	1
Less than 12 Months | security	4	0	0
12 Months or Greater | security	1	1	1
Fair Value
Total	$ 67.5	$ 12.0	$ 11.1
Less than 12 Months	56.7	0.0	0.0
12 Months or Greater	10.8	12.0	11.1
Unrealized Losses
Total	(5.5)	(0.5)	(1.4)
Less than 12 Months	(3.8)	0.0	0.0
12 Months or Greater	$ (1.7)	$ (0.5)	$ (1.4)