Investments Gross Unrealized losses by Major Security (Detail) $ in Millions	Sep. 30, 2019 USD ($) security	Dec. 31, 2018 USD ($) security	Sep. 30, 2018 USD ($) security
Number of Securities
Total | security	243	1,292	1,526
Less than 12 Months | security	90	349	723
12 Months or Greater | security	153	943	803
Fair Value
Total	$ 4,812.7	$ 17,652.2	$ 22,879.8
Less than 12 Months	3,617.6	5,493.2	12,985.1
12 Months or Greater	1,195.1	12,159.0	9,894.7
Unrealized Losses
Total	(17.2)	(249.6)	(349.0)
Less than 12 Months	(12.1)	(47.3)	(131.9)
12 Months or Greater	$ (5.1)	$ (202.3)	$ (217.1)
U.S. government obligations
Number of Securities
Total | security	14	51	91
Less than 12 Months | security	7	2	39
12 Months or Greater | security	7	49	52
Fair Value
Total	$ 2,708.0	$ 4,438.0	$ 9,752.5
Less than 12 Months	2,534.4	126.6	5,025.0
12 Months or Greater	173.6	4,311.4	4,727.5
Unrealized Losses
Total	(9.2)	(52.1)	(189.5)
Less than 12 Months	(8.9)	(0.1)	(67.2)
12 Months or Greater	$ (0.3)	$ (52.0)	$ (122.3)
State and local government obligations
Number of Securities
Total | security	60	299	459
Less than 12 Months | security	25	49	227
12 Months or Greater | security	35	250	232
Fair Value
Total	$ 228.3	$ 972.4	$ 1,394.3
Less than 12 Months	100.4	192.7	680.5
12 Months or Greater	127.9	779.7	713.8
Unrealized Losses
Total	(1.1)	(12.8)	(23.2)
Less than 12 Months	(0.7)	(0.3)	(6.4)
12 Months or Greater	$ (0.4)	$ (12.5)	$ (16.8)
Corporate debt securities
Number of Securities
Total | security	24	368	369
Less than 12 Months | security	5	133	221
12 Months or Greater | security	19	235	148
Fair Value
Total	$ 245.5	$ 6,723.3	$ 5,959.2
Less than 12 Months	77.2	2,613.3	3,963.9
12 Months or Greater	168.3	4,110.0	1,995.3
Unrealized Losses
Total	(0.4)	(125.3)	(85.2)
Less than 12 Months	(0.1)	(33.4)	(45.3)
12 Months or Greater	$ (0.3)	$ (91.9)	$ (39.9)
Residential mortgage-backed securities
Number of Securities
Total | security	39	228	234
Less than 12 Months | security	5	32	44
12 Months or Greater | security	34	196	190
Fair Value
Total	$ 301.0	$ 450.2	$ 521.7
Less than 12 Months	138.7	248.8	325.0
12 Months or Greater	162.3	201.4	196.7
Unrealized Losses
Total	(1.5)	(5.1)	(5.6)
Less than 12 Months	(0.1)	(0.8)	(0.6)
12 Months or Greater	$ (1.4)	$ (4.3)	$ (5.0)
Commercial mortgage-backed securities
Number of Securities
Total | security	48	140	151
Less than 12 Months | security	26	48	80
12 Months or Greater | security	22	92	71
Fair Value
Total	$ 743.3	$ 2,328.5	$ 2,392.7
Less than 12 Months	361.9	741.2	1,164.2
12 Months or Greater	381.4	1,587.3	1,228.5
Unrealized Losses
Total	(1.9)	(39.0)	(30.5)
Less than 12 Months	(0.8)	(8.9)	(8.3)
12 Months or Greater	$ (1.1)	$ (30.1)	$ (22.2)
Other asset-backed securities
Number of Securities
Total | security	55	203	219
Less than 12 Months | security	20	84	111
12 Months or Greater | security	35	119	108
Fair Value
Total	$ 529.4	$ 2,691.3	$ 2,828.6
Less than 12 Months	359.1	1,551.7	1,821.8
12 Months or Greater	170.3	1,139.6	1,006.8
Unrealized Losses
Total	(0.6)	(11.8)	(13.6)
Less than 12 Months	(0.2)	(3.2)	(4.0)
12 Months or Greater	$ (0.4)	$ (8.6)	$ (9.6)
Redeemable preferred stocks
Number of Securities
Total | security	3	3	3
Less than 12 Months | security	2	1	1
12 Months or Greater | security	1	2	2
Fair Value
Total	$ 57.2	$ 48.5	$ 30.8
Less than 12 Months	45.9	18.9	4.7
12 Months or Greater	11.3	29.6	26.1
Unrealized Losses
Total	(2.5)	(3.5)	(1.4)
Less than 12 Months	(1.3)	(0.6)	(0.1)
12 Months or Greater	$ (1.2)	$ (2.9)	$ (1.3)