Investments Gross Unrealized losses by Major Security (Detail) $ in Millions	Mar. 31, 2021 USD ($) security	Dec. 31, 2020 USD ($) security	Mar. 31, 2020 USD ($) security
Number of Securities
Total | security	367	133	817
Less than 12 Months | security	322	64	749
12 Months or Greater | security	45	69	68
Fair Value
Total	$ 14,361.5	$ 2,981.5	$ 13,422.7
Less than 12 Months	13,708.9	2,017.1	12,907.2
12 Months or Greater	652.6	964.4	515.5
Unrealized Losses
Total	(199.6)	(17.3)	(377.2)
Less than 12 Months	(192.0)	(4.9)	(353.0)
12 Months or Greater	$ (7.6)	$ (12.4)	$ (24.2)
U.S. government obligations
Number of Securities
Total | security	36	9	1
Less than 12 Months | security	35	9	1
12 Months or Greater | security	1	0	0
Fair Value
Total	$ 8,612.2	$ 1,511.0	$ 100.3
Less than 12 Months	8,598.6	1,511.0	100.3
12 Months or Greater	13.6	0.0	0.0
Unrealized Losses
Total	(111.0)	(3.7)	0.0
Less than 12 Months	(109.7)	(3.7)	0.0
12 Months or Greater	$ (1.3)	$ 0.0	$ 0.0
State and local government obligations
Number of Securities
Total | security	92	30	89
Less than 12 Months | security	92	30	78
12 Months or Greater | security	0	0	11
Fair Value
Total	$ 816.0	$ 208.7	$ 528.4
Less than 12 Months	816.0	208.7	507.7
12 Months or Greater	0.0	0.0	20.7
Unrealized Losses
Total	(16.0)	(0.7)	(4.5)
Less than 12 Months	(16.0)	(0.7)	(4.4)
12 Months or Greater	$ 0.0	$ 0.0	$ (0.1)
Corporate debt securities
Number of Securities
Total | security	79	7	233
Less than 12 Months | security	76	7	233
12 Months or Greater | security	3	0	0
Fair Value
Total	$ 1,751.4	$ 129.4	$ 4,205.6
Less than 12 Months	1,698.4	129.4	4,205.6
12 Months or Greater	53.0	0.0	0.0
Unrealized Losses
Total	(24.0)	(0.1)	(120.1)
Less than 12 Months	(23.9)	(0.1)	(120.1)
12 Months or Greater	$ (0.1)	$ 0.0	$ 0.0
Residential mortgage-backed securities
Number of Securities
Total | security	21	21	70
Less than 12 Months | security	6	0	46
12 Months or Greater | security	15	21	24
Fair Value
Total	$ 123.4	$ 44.4	$ 532.5
Less than 12 Months	94.4	0.0	425.0
12 Months or Greater	29.0	44.4	107.5
Unrealized Losses
Total	(0.8)	(0.9)	(19.2)
Less than 12 Months	(0.2)	0.0	(11.9)
12 Months or Greater	$ (0.6)	$ (0.9)	$ (7.3)
Commercial mortgage-backed securities
Number of Securities
Total | security	95	43	204
Less than 12 Months | security	77	9	187
12 Months or Greater | security	18	34	17
Fair Value
Total	$ 2,542.2	$ 893.3	$ 4,441.5
Less than 12 Months	2,023.9	93.6	4,109.2
12 Months or Greater	518.3	799.7	332.3
Unrealized Losses
Total	(45.1)	(10.0)	(188.3)
Less than 12 Months	(41.7)	(0.3)	(173.8)
12 Months or Greater	$ (3.4)	$ (9.7)	$ (14.5)
Other asset-backed securities
Number of Securities
Total | security	43	22	216
Less than 12 Months | security	36	9	201
12 Months or Greater | security	7	13	15
Fair Value
Total	$ 505.8	$ 183.7	$ 3,558.1
Less than 12 Months	477.6	74.4	3,514.2
12 Months or Greater	28.2	109.3	43.9
Unrealized Losses
Total	(0.7)	(0.5)	(40.0)
Less than 12 Months	(0.5)	(0.1)	(39.0)
12 Months or Greater	$ (0.2)	$ (0.4)	$ (1.0)
Redeemable preferred stocks
Number of Securities
Total | security	1	1	4
Less than 12 Months | security	0	0	3
12 Months or Greater | security	1	1	1
Fair Value
Total	$ 10.5	$ 11.0	$ 56.3
Less than 12 Months	0.0	0.0	45.2
12 Months or Greater	10.5	11.0	11.1
Unrealized Losses
Total	(2.0)	(1.4)	(5.1)
Less than 12 Months	0.0	0.0	(3.8)
12 Months or Greater	$ (2.0)	$ (1.4)	$ (1.3)