Investments Gross Unrealized losses by Major Security (Detail) $ in Millions	Jun. 30, 2019 USD ($) security	Dec. 31, 2018 USD ($) security	Jun. 30, 2018 USD ($) security
Number of Securities
Total | security	363	1,292	1,407
Less than 12 Months | security	73	349	860
12 Months or Greater | security	290	943	547
Fair Value
Total	$ 3,768.4	$ 17,652.2	$ 19,898.7
Less than 12 Months	966.5	5,493.2	14,234.4
12 Months or Greater	2,801.9	12,159.0	5,664.3
Unrealized Losses
Total	(11.9)	(249.6)	(304.1)
Less than 12 Months	(2.0)	(47.3)	(181.9)
12 Months or Greater	$ (9.9)	$ (202.3)	$ (122.2)
U.S. government obligations
Number of Securities
Total | security	16	51	70
Less than 12 Months | security	0	2	33
12 Months or Greater | security	16	49	37
Fair Value
Total	$ 572.8	$ 4,438.0	$ 7,503.2
Less than 12 Months	0.0	126.6	4,543.5
12 Months or Greater	572.8	4,311.4	2,959.7
Unrealized Losses
Total	(2.2)	(52.1)	(141.3)
Less than 12 Months	0.0	(0.1)	(73.6)
12 Months or Greater	$ (2.2)	$ (52.0)	$ (67.7)
State and local government obligations
Number of Securities
Total | security	85	299	355
Less than 12 Months | security	11	49	219
12 Months or Greater | security	74	250	136
Fair Value
Total	$ 398.3	$ 972.4	$ 1,050.9
Less than 12 Months	133.6	192.7	631.9
12 Months or Greater	264.7	779.7	419.0
Unrealized Losses
Total	(1.3)	(12.8)	(15.9)
Less than 12 Months	(0.1)	(0.3)	(7.1)
12 Months or Greater	$ (1.2)	$ (12.5)	$ (8.8)
Corporate debt securities
Number of Securities
Total | security	68	368	391
Less than 12 Months | security	4	133	322
12 Months or Greater | security	64	235	69
Fair Value
Total	$ 928.9	$ 6,723.3	$ 6,153.5
Less than 12 Months	42.3	2,613.3	5,439.3
12 Months or Greater	886.6	4,110.0	714.2
Unrealized Losses
Total	(1.8)	(125.3)	(94.3)
Less than 12 Months	(0.1)	(33.4)	(77.2)
12 Months or Greater	$ (1.7)	$ (91.9)	$ (17.1)
Residential mortgage-backed securities
Number of Securities
Total | security	43	228	227
Less than 12 Months | security	8	32	52
12 Months or Greater | security	35	196	175
Fair Value
Total	$ 203.8	$ 450.2	$ 380.3
Less than 12 Months	24.5	248.8	188.6
12 Months or Greater	179.3	201.4	191.7
Unrealized Losses
Total	(1.4)	(5.1)	(5.4)
Less than 12 Months	0.0	(0.8)	(0.9)
12 Months or Greater	$ (1.4)	$ (4.3)	$ (4.5)
Commercial mortgage-backed securities
Number of Securities
Total | security	57	140	142
Less than 12 Months | security	26	48	85
12 Months or Greater | security	31	92	57
Fair Value
Total	$ 963.7	$ 2,328.5	$ 2,300.9
Less than 12 Months	485.5	741.2	1,371.4
12 Months or Greater	478.2	1,587.3	929.5
Unrealized Losses
Total	(2.6)	(39.0)	(31.5)
Less than 12 Months	(1.1)	(8.9)	(14.4)
12 Months or Greater	$ (1.5)	$ (30.1)	$ (17.1)
Other asset-backed securities
Number of Securities
Total | security	92	203	219
Less than 12 Months | security	23	84	148
12 Months or Greater | security	69	119	71
Fair Value
Total	$ 674.6	$ 2,691.3	$ 2,479.7
Less than 12 Months	265.6	1,551.7	2,055.1
12 Months or Greater	409.0	1,139.6	424.6
Unrealized Losses
Total	(1.0)	(11.8)	(13.7)
Less than 12 Months	(0.2)	(3.2)	(8.5)
12 Months or Greater	$ (0.8)	$ (8.6)	$ (5.2)
Redeemable preferred stocks
Number of Securities
Total | security	2	3	3
Less than 12 Months | security	1	1	1
12 Months or Greater | security	1	2	2
Fair Value
Total	$ 26.3	$ 48.5	$ 30.2
Less than 12 Months	15.0	18.9	4.6
12 Months or Greater	11.3	29.6	25.6
Unrealized Losses
Total	(1.6)	(3.5)	(2.0)
Less than 12 Months	(0.5)	(0.6)	(0.2)
12 Months or Greater	$ (1.1)	$ (2.9)	$ (1.8)