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	Mar. 31, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	$ 1,551.4	$ 701.2
Unrealized Losses, Less Than One Year	120.6	23.0
Fair Value, One Year or Greater	98.2	253.5
Unrealized Losses, One Year or Greater	27.7	17.0
Fair Value, Total	1,649.6	954.7
Unrealized Losses, Total	148.3	40.0
U.S. Governments [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	1.4	114.6
Unrealized Losses, Less Than One Year	0.0	1.1
Fair Value, One Year or Greater	0.0	17.0
Unrealized Losses, One Year or Greater	0.0	0.1
Fair Value, Total	1.4	131.6
Unrealized Losses, Total	0.0	1.2
Foreign Governments [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	143.6	117.6
Unrealized Losses, Less Than One Year	10.4	0.7
Fair Value, One Year or Greater	0.0	5.1
Unrealized Losses, One Year or Greater	0.0	0.0
Fair Value, Total	143.6	122.7
Unrealized Losses, Total	10.4	0.7
Obligations of States and Political Subdivisions [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	24.2	0.7
Unrealized Losses, Less Than One Year	0.7	0.0
Fair Value, One Year or Greater	0.0	2.1
Unrealized Losses, One Year or Greater	0.0	0.1
Fair Value, Total	24.2	2.8
Unrealized Losses, Total	0.7	0.1
Corporate Bonds [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	850.3	249.4
Unrealized Losses, Less Than One Year	75.7	18.9
Fair Value, One Year or Greater	29.7	63.6
Unrealized Losses, One Year or Greater	22.6	15.8
Fair Value, Total	880.0	313.0
Unrealized Losses, Total	98.3	34.7
Commercial Mortgage-backed Securities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	132.6	74.8
Unrealized Losses, Less Than One Year	3.5	1.1
Fair Value, One Year or Greater	0.0	4.9
Unrealized Losses, One Year or Greater	0.0	0.0
Fair Value, Total	132.6	79.7
Unrealized Losses, Total	3.5	1.1
Residential Mortgage-backed Securities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	101.1	66.9
Unrealized Losses, Less Than One Year	5.5	0.3
Fair Value, One Year or Greater	8.9	25.2
Unrealized Losses, One Year or Greater	0.9	0.3
Fair Value, Total	110.0	92.1
Unrealized Losses, Total	6.4	0.6
Asset-backed Securities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	103.3	22.5
Unrealized Losses, Less Than One Year	5.8	0.1
Fair Value, One Year or Greater	4.7	18.9
Unrealized Losses, One Year or Greater	0.4	0.1
Fair Value, Total	108.0	41.4
Unrealized Losses, Total	6.2	0.2
Collateralized Debt Obligations [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	194.9	54.7
Unrealized Losses, Less Than One Year	19.0	0.8
Fair Value, One Year or Greater	54.9	116.7
Unrealized Losses, One Year or Greater	3.8	0.6
Fair Value, Total	249.8	171.4
Unrealized Losses, Total	$ 22.8	$ 1.4