Investments Gross Unrealized losses by Major Security (Detail) $ in Millions	Jun. 30, 2021 USD ($) security	Dec. 31, 2020 USD ($) security	Jun. 30, 2020 USD ($) security
Number of Securities
Total | security	697	133	225
Less than 12 Months | security	653	64	174
12 Months or Greater | security	44	69	51
Fair Value
Total	$ 14,757.0	$ 2,981.5	$ 3,615.9
Less than 12 Months	13,850.6	2,017.1	3,043.9
12 Months or Greater	906.4	964.4	572.0
Unrealized Losses
Total	(101.6)	(17.3)	(68.4)
Less than 12 Months	(91.4)	(4.9)	(57.5)
12 Months or Greater	$ (10.2)	$ (12.4)	$ (10.9)
U.S. government obligations
Number of Securities
Total | security	59	9	4
Less than 12 Months | security	55	9	4
12 Months or Greater | security	4	0	0
Fair Value
Total	$ 10,629.4	$ 1,511.0	$ 120.6
Less than 12 Months	10,162.4	1,511.0	120.6
12 Months or Greater	467.0	0.0	0.0
Unrealized Losses
Total	(72.9)	(3.7)	(0.1)
Less than 12 Months	(66.8)	(3.7)	(0.1)
12 Months or Greater	$ (6.1)	$ 0.0	$ 0.0
State and local government obligations
Number of Securities
Total | security	81	30	17
Less than 12 Months | security	72	30	16
12 Months or Greater | security	9	0	1
Fair Value
Total	$ 436.2	$ 208.7	$ 135.0
Less than 12 Months	400.0	208.7	131.2
12 Months or Greater	36.2	0.0	3.8
Unrealized Losses
Total	(3.3)	(0.7)	(0.6)
Less than 12 Months	(2.6)	(0.7)	(0.6)
12 Months or Greater	$ (0.7)	$ 0.0	$ 0.0
Corporate debt securities
Number of Securities
Total | security	292	7	21
Less than 12 Months | security	288	7	21
12 Months or Greater | security	4	0	0
Fair Value
Total	$ 1,142.5	$ 129.4	$ 292.8
Less than 12 Months	1,065.2	129.4	292.8
12 Months or Greater	77.3	0.0	0.0
Unrealized Losses
Total	(9.9)	(0.1)	(3.4)
Less than 12 Months	(9.5)	(0.1)	(3.4)
12 Months or Greater	$ (0.4)	$ 0.0	$ 0.0
Residential mortgage-backed securities
Number of Securities
Total | security	77	21	40
Less than 12 Months | security	64	0	21
12 Months or Greater | security	13	21	19
Fair Value
Total	$ 192.2	$ 44.4	$ 188.2
Less than 12 Months	172.4	0.0	93.7
12 Months or Greater	19.8	44.4	94.5
Unrealized Losses
Total	(1.1)	(0.9)	(3.4)
Less than 12 Months	(0.7)	0.0	(1.7)
12 Months or Greater	$ (0.4)	$ (0.9)	$ (1.7)
Commercial mortgage-backed securities
Number of Securities
Total | security	68	43	113
Less than 12 Months | security	61	9	91
12 Months or Greater | security	7	34	22
Fair Value
Total	$ 1,272.0	$ 893.3	$ 2,536.6
Less than 12 Months	1,006.9	93.6	2,106.6
12 Months or Greater	265.1	799.7	430.0
Unrealized Losses
Total	(11.5)	(10.0)	(55.8)
Less than 12 Months	(10.5)	(0.3)	(48.3)
12 Months or Greater	$ (1.0)	$ (9.7)	$ (7.5)
Other asset-backed securities
Number of Securities
Total | security	119	22	29
Less than 12 Months | security	113	9	21
12 Months or Greater | security	6	13	8
Fair Value
Total	$ 1,073.6	$ 183.7	$ 331.7
Less than 12 Months	1,043.7	74.4	299.0
12 Months or Greater	29.9	109.3	32.7
Unrealized Losses
Total	(1.5)	(0.5)	(3.7)
Less than 12 Months	(1.3)	(0.1)	(3.4)
12 Months or Greater	$ (0.2)	$ (0.4)	$ (0.3)
Redeemable preferred stocks
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	$ 11.1	$ 11.0	$ 11.0
Less than 12 Months	0.0	0.0	0.0
12 Months or Greater	11.1	11.0	11.0
Unrealized Losses
Total	(1.4)	(1.4)	(1.4)
Less than 12 Months	0.0	0.0	0.0
12 Months or Greater	$ (1.4)	$ (1.4)	$ (1.4)