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	Jun. 30, 2019	Dec. 31, 2018
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	$ 579.7	$ 1,514.3
Unrealized Losses, Less Than One Year	22.2	55.3
Fair Value, One Year or Greater	605.4	1,105.3
Unrealized Losses, One Year or Greater	7.2	30.9
Fair Value, Total	1,185.1	2,619.6
Unrealized Losses, Total	29.4	86.2
U.S. Governments [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	5.4	28.2
Unrealized Losses, Less Than One Year	0.0	0.2
Fair Value, One Year or Greater	90.6	173.0
Unrealized Losses, One Year or Greater	0.7	4.7
Fair Value, Total	96.0	201.2
Unrealized Losses, Total	0.7	4.9
Foreign Governments [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	61.8	73.4
Unrealized Losses, Less Than One Year	1.3	3.6
Fair Value, One Year or Greater	57.5	125.0
Unrealized Losses, One Year or Greater	0.5	4.2
Fair Value, Total	119.3	198.4
Unrealized Losses, Total	1.8	7.8
Obligations of States and Political Subdivisions [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	0.5	53.3
Unrealized Losses, Less Than One Year	0.0	0.6
Fair Value, One Year or Greater	4.3	25.3
Unrealized Losses, One Year or Greater	0.1	0.6
Fair Value, Total	4.8	78.6
Unrealized Losses, Total	0.1	1.2
Corporate Bonds [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	306.4	964.3
Unrealized Losses, Less Than One Year	19.3	45.7
Fair Value, One Year or Greater	252.6	440.8
Unrealized Losses, One Year or Greater	4.2	13.0
Fair Value, Total	559.0	1,405.1
Unrealized Losses, Total	23.5	58.7
Commercial Mortgage-backed Securities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	10.2	48.5
Unrealized Losses, Less Than One Year	0.0	0.6
Fair Value, One Year or Greater	23.1	90.6
Unrealized Losses, One Year or Greater	0.2	2.6
Fair Value, Total	33.3	139.1
Unrealized Losses, Total	0.2	3.2
Residential Mortgage-backed Securities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	30.0	63.5
Unrealized Losses, Less Than One Year	0.3	0.7
Fair Value, One Year or Greater	87.3	176.1
Unrealized Losses, One Year or Greater	1.0	5.0
Fair Value, Total	117.3	239.6
Unrealized Losses, Total	1.3	5.7
Asset-backed Securities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	13.7	73.6
Unrealized Losses, Less Than One Year	0.1	0.6
Fair Value, One Year or Greater	42.1	64.2
Unrealized Losses, One Year or Greater	0.2	0.6
Fair Value, Total	55.8	137.8
Unrealized Losses, Total	0.3	1.2
Collateralized Debt Obligations [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	151.7	209.5
Unrealized Losses, Less Than One Year	1.2	3.3
Fair Value, One Year or Greater	47.9	10.3
Unrealized Losses, One Year or Greater	0.3	0.2
Fair Value, Total	199.6	219.8
Unrealized Losses, Total	$ 1.5	$ 3.5