Investments - Schedule of Aging of Unrealized Losses on Company's Investments in Fixed Maturities, Equity Securities and Other Investments (Detail) - Fixed Maturities [Member] - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	$ 807.5	$ 1,514.3
Unrealized Losses, Less Than One Year	37.4	55.3
Fair Value, One Year or Greater	253.5	1,105.3
Unrealized Losses, One Year or Greater	8.2	30.9
Fair Value, Total	1,061.0	2,619.6
Unrealized Losses, Total	45.6	86.2
U.S. Governments [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	75.0	28.2
Unrealized Losses, Less Than One Year	0.4	0.2
Fair Value, One Year or Greater	18.8	173.0
Unrealized Losses, One Year or Greater	0.1	4.7
Fair Value, Total	93.8	201.2
Unrealized Losses, Total	0.5	4.9
Foreign Governments [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	108.2	73.4
Unrealized Losses, Less Than One Year	2.1	3.6
Fair Value, One Year or Greater	9.6	125.0
Unrealized Losses, One Year or Greater	0.2	4.2
Fair Value, Total	117.8	198.4
Unrealized Losses, Total	2.3	7.8
Obligations of States and Political Subdivisions [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	1.1	53.3
Unrealized Losses, Less Than One Year	0.0	0.6
Fair Value, One Year or Greater	2.4	25.3
Unrealized Losses, One Year or Greater	0.1	0.6
Fair Value, Total	3.5	78.6
Unrealized Losses, Total	0.1	1.2
Corporate Bonds [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	355.6	964.3
Unrealized Losses, Less Than One Year	32.3	45.7
Fair Value, One Year or Greater	100.0	440.8
Unrealized Losses, One Year or Greater	6.9	13.0
Fair Value, Total	455.6	1,405.1
Unrealized Losses, Total	39.2	58.7
Commercial Mortgage-backed Securities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	53.8	48.5
Unrealized Losses, Less Than One Year	0.2	0.6
Fair Value, One Year or Greater	5.3	90.6
Unrealized Losses, One Year or Greater	0.1	2.6
Fair Value, Total	59.1	139.1
Unrealized Losses, Total	0.3	3.2
Residential Mortgage-backed Securities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	57.1	63.5
Unrealized Losses, Less Than One Year	0.4	0.7
Fair Value, One Year or Greater	43.9	176.1
Unrealized Losses, One Year or Greater	0.5	5.0
Fair Value, Total	101.0	239.6
Unrealized Losses, Total	0.9	5.7
Asset-backed Securities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	17.9	73.6
Unrealized Losses, Less Than One Year	0.2	0.6
Fair Value, One Year or Greater	19.4	64.2
Unrealized Losses, One Year or Greater	0.1	0.6
Fair Value, Total	37.3	137.8
Unrealized Losses, Total	0.3	1.2
Collateralized Debt Obligations [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	138.8	209.5
Unrealized Losses, Less Than One Year	1.8	3.3
Fair Value, One Year or Greater	54.1	10.3
Unrealized Losses, One Year or Greater	0.2	0.2
Fair Value, Total	192.9	219.8
Unrealized Losses, Total	$ 2.0	$ 3.5