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	Dec. 31, 2019	Dec. 31, 2018
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	$ 701.2	$ 1,514.3
Unrealized Losses, Less Than One Year	23.0	55.3
Fair Value, One Year or Greater	253.5	1,105.3
Unrealized Losses, One Year or Greater	17.0	30.9
Fair Value, Total	954.7	2,619.6
Unrealized Losses, Total	40.0	86.2
U.S. Governments [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	114.6	28.2
Unrealized Losses, Less Than One Year	1.1	0.2
Fair Value, One Year or Greater	17.0	173.0
Unrealized Losses, One Year or Greater	0.1	4.7
Fair Value, Total	131.6	201.2
Unrealized Losses, Total	1.2	4.9
Foreign Governments [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	117.6	73.4
Unrealized Losses, Less Than One Year	0.7	3.6
Fair Value, One Year or Greater	5.1	125.0
Unrealized Losses, One Year or Greater	0.0	4.2
Fair Value, Total	122.7	198.4
Unrealized Losses, Total	0.7	7.8
Obligations of States and Political Subdivisions [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	0.7	53.3
Unrealized Losses, Less Than One Year	0.0	0.6
Fair Value, One Year or Greater	2.1	25.3
Unrealized Losses, One Year or Greater	0.1	0.6
Fair Value, Total	2.8	78.6
Unrealized Losses, Total	0.1	1.2
Corporate Bonds [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	249.4	964.3
Unrealized Losses, Less Than One Year	18.9	45.7
Fair Value, One Year or Greater	63.6	440.8
Unrealized Losses, One Year or Greater	15.8	13.0
Fair Value, Total	313.0	1,405.1
Unrealized Losses, Total	34.7	58.7
Commercial Mortgage-backed Securities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	74.8	48.5
Unrealized Losses, Less Than One Year	1.1	0.6
Fair Value, One Year or Greater	4.9	90.6
Unrealized Losses, One Year or Greater	0.0	2.6
Fair Value, Total	79.7	139.1
Unrealized Losses, Total	1.1	3.2
Residential Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	66.9	63.5
Unrealized Losses, Less Than One Year	0.3	0.7
Fair Value, One Year or Greater	25.2	176.1
Unrealized Losses, One Year or Greater	0.3	5.0
Fair Value, Total	92.1	239.6
Unrealized Losses, Total	0.6	5.7
Asset-backed Securities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	22.5	73.6
Unrealized Losses, Less Than One Year	0.1	0.6
Fair Value, One Year or Greater	18.9	64.2
Unrealized Losses, One Year or Greater	0.1	0.6
Fair Value, Total	41.4	137.8
Unrealized Losses, Total	0.2	1.2
Collateralized Debt Obligations [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	54.7	209.5
Unrealized Losses, Less Than One Year	0.8	3.3
Fair Value, One Year or Greater	116.7	10.3
Unrealized Losses, One Year or Greater	0.6	0.2
Fair Value, Total	171.4	219.8
Unrealized Losses, Total	$ 1.4	$ 3.5