Investments Gross Unrealized losses by Major Security (Detail) $ in Millions	Sep. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	554	1,167	1,030
Total Fair Value	$ 3,507.1	$ 9,725.1	$ 6,255.9
Gross Unrealized Losses	$ (45.1)	$ (166.7)	$ (127.8)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year	313	993	882
Less than 12 Months Fair Value	$ 1,640.2	$ 7,842.3	$ 4,553.2
Less than 12 Months Unrealized Losses	$ (8.7)	$ (93.9)	$ (76.4)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year	241	174	148
12 Months or Greater Fair Value	$ 1,866.9	$ 1,882.8	$ 1,702.7
12 Months or Greater Unrealized Losses	$ (36.4)	$ (72.8)	$ (51.4)
Fixed maturities
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	461	1,093	880
Total Fair Value	$ 3,266.2	$ 9,258.5	$ 5,667.4
Gross Unrealized Losses	$ (26.8)	$ (136.8)	$ (78.0)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year	229	928	741
Less than 12 Months Fair Value	$ 1,580.8	$ 7,556.7	$ 4,146.5
Less than 12 Months Unrealized Losses	$ (4.3)	$ (78.0)	$ (39.5)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year	232	165	139
12 Months or Greater Fair Value	$ 1,685.4	$ 1,701.8	$ 1,520.9
12 Months or Greater Unrealized Losses	$ (22.5)	$ (58.8)	$ (38.5)
Fixed maturities | U.S. government obligations
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	4	22	0
Total Fair Value	$ 104.8	$ 897.1	$ 0.0
Gross Unrealized Losses	$ (0.3)	$ (0.6)	$ 0.0
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year	4	22	0
Less than 12 Months Fair Value	$ 104.8	$ 897.1	$ 0.0
Less than 12 Months Unrealized Losses	$ (0.3)	$ (0.6)	$ 0.0
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year	0	0	0
12 Months or Greater Fair Value	$ 0.0	$ 0.0	$ 0.0
12 Months or Greater Unrealized Losses	$ 0.0	$ 0.0	$ 0.0
Fixed maturities | State and local government obligations
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	151	290	349
Total Fair Value	$ 512.0	$ 606.7	$ 637.1
Gross Unrealized Losses	$ (1.6)	$ (3.7)	$ (4.2)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year	117	264	328
Less than 12 Months Fair Value	$ 409.6	$ 500.7	$ 551.4
Less than 12 Months Unrealized Losses	$ (1.0)	$ (2.6)	$ (3.7)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year	34	26	21
12 Months or Greater Fair Value	$ 102.4	$ 106.0	$ 85.7
12 Months or Greater Unrealized Losses	$ (0.6)	$ (1.1)	$ (0.5)
Fixed maturities | Corporate debt securities
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	52	215	140
Total Fair Value	$ 598.3	$ 2,580.6	$ 1,596.5
Gross Unrealized Losses	$ (1.8)	$ (33.0)	$ (21.4)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year	42	197	125
Less than 12 Months Fair Value	$ 541.5	$ 2,294.6	$ 1,292.1
Less than 12 Months Unrealized Losses	$ (0.8)	$ (25.2)	$ (15.7)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year	10	18	15
12 Months or Greater Fair Value	$ 56.8	$ 286.0	$ 304.4
12 Months or Greater Unrealized Losses	$ (1.0)	$ (7.8)	$ (5.7)
Fixed maturities | Residential mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	138	188	141
Total Fair Value	$ 915.8	$ 1,294.7	$ 1,205.9
Gross Unrealized Losses	$ (15.5)	$ (20.6)	$ (18.4)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year	26	115	76
Less than 12 Months Fair Value	$ 108.8	$ 493.4	$ 535.8
Less than 12 Months Unrealized Losses	$ (0.4)	$ (3.7)	$ (4.1)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year	112	73	65
12 Months or Greater Fair Value	$ 807.0	$ 801.3	$ 670.1
12 Months or Greater Unrealized Losses	$ (15.1)	$ (16.9)	$ (14.3)
Fixed maturities | Agency residential pass-through obligations
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	15	61	54
Total Fair Value	$ 4.2	$ 84.9	$ 67.6
Gross Unrealized Losses	$ 0.0	$ (1.1)	$ (0.4)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year	7	61	54
Less than 12 Months Fair Value	$ 1.9	$ 84.9	$ 67.6
Less than 12 Months Unrealized Losses	$ 0.0	$ (1.1)	$ (0.4)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year	8	0	0
12 Months or Greater Fair Value	$ 2.3	$ 0.0	$ 0.0
12 Months or Greater Unrealized Losses	$ 0.0	$ 0.0	$ 0.0
Fixed maturities | Commercial mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	56	207	127
Total Fair Value	$ 610.6	$ 2,046.5	$ 1,100.0
Gross Unrealized Losses	$ (6.1)	$ (29.4)	$ (9.3)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year	17	171	99
Less than 12 Months Fair Value	$ 194.7	$ 1,694.6	$ 813.8
Less than 12 Months Unrealized Losses	$ (1.7)	$ (25.8)	$ (7.6)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year	39	36	28
12 Months or Greater Fair Value	$ 415.9	$ 351.9	$ 286.2
12 Months or Greater Unrealized Losses	$ (4.4)	$ (3.6)	$ (1.7)
Fixed maturities | Other asset-backed securities
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	43	101	60
Total Fair Value	$ 488.7	$ 1,548.6	$ 845.5
Gross Unrealized Losses	$ (0.3)	$ (5.1)	$ (1.5)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year	16	92	53
Less than 12 Months Fair Value	$ 219.5	$ 1,472.0	$ 764.0
Less than 12 Months Unrealized Losses	$ (0.1)	$ (4.5)	$ (0.9)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year	27	9	7
12 Months or Greater Fair Value	$ 269.2	$ 76.6	$ 81.5
12 Months or Greater Unrealized Losses	$ (0.2)	$ (0.6)	$ (0.6)
Fixed maturities | Redeemable preferred stocks
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	2	9	9
Total Fair Value	$ 31.8	$ 199.4	$ 214.8
Gross Unrealized Losses	$ (1.2)	$ (43.3)	$ (22.8)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year	0	6	6
Less than 12 Months Fair Value	$ 0.0	$ 119.4	$ 121.8
Less than 12 Months Unrealized Losses	$ 0.0	$ (14.5)	$ (7.1)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year	2	3	3
12 Months or Greater Fair Value	$ 31.8	$ 80.0	$ 93.0
12 Months or Greater Unrealized Losses	$ (1.2)	$ (28.8)	$ (15.7)
Equity securities
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	93	74	150
Total Fair Value	$ 240.9	$ 466.6	$ 588.5
Gross Unrealized Losses	$ (18.3)	$ (29.9)	$ (49.8)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year	84	65	141
Less than 12 Months Fair Value	$ 59.4	$ 285.6	$ 406.7
Less than 12 Months Unrealized Losses	$ (4.4)	$ (15.9)	$ (36.9)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year	9	9	9
12 Months or Greater Fair Value	$ 181.5	$ 181.0	$ 181.8
12 Months or Greater Unrealized Losses	$ (13.9)	$ (14.0)	$ (12.9)
Equity securities | Nonredeemable preferred stocks
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	7	10	14
Total Fair Value	$ 187.5	$ 301.8	$ 376.9
Gross Unrealized Losses	$ (13.9)	$ (15.7)	$ (14.8)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year	1	5	9
Less than 12 Months Fair Value	$ 9.9	$ 124.2	$ 197.6
Less than 12 Months Unrealized Losses	$ (0.2)	$ (1.7)	$ (2.5)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year	6	5	5
12 Months or Greater Fair Value	$ 177.6	$ 177.6	$ 179.3
12 Months or Greater Unrealized Losses	$ (13.7)	$ (14.0)	$ (12.3)
Equity securities | Common equities
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	86	64	136
Total Fair Value	$ 53.4	$ 164.8	$ 211.6
Gross Unrealized Losses	$ (4.4)	$ (14.2)	$ (35.0)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year	83	60	132
Less than 12 Months Fair Value	$ 49.5	$ 161.4	$ 209.1
Less than 12 Months Unrealized Losses	$ (4.2)	$ (14.2)	$ (34.4)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year	3	4	4
12 Months or Greater Fair Value	$ 3.9	$ 3.4	$ 2.5
12 Months or Greater Unrealized Losses	$ (0.2)	$ 0.0	$ (0.6)