Investments - Schedule of Aging of Unrealized Losses on Company's Investments in Fixed Maturities, Equity Securities and Other Investments (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Unrealized losses one year or greater, less than		$ 0.1
Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	$ 2,584.9	1,538.9
Unrealized Losses, Less Than One Year	56.3	17.2
Fair Value, One Year or Greater	95.2	342.7
Unrealized Losses, One Year or Greater	5.7	10.1
Fair Value, Total	2,680.1	1,881.6
Unrealized Losses, Total	62.0	27.3
Fixed Maturities [Member] | U.S. Governments [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	260.0	313.7
Unrealized Losses, Less Than One Year	7.2	1.9
Fair Value, One Year or Greater	17.1	83.7
Unrealized Losses, One Year or Greater	0.9	3.1
Fair Value, Total	277.1	397.4
Unrealized Losses, Total	8.1	5.0
Fixed Maturities [Member] | Foreign Governments [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	229.5	175.2
Unrealized Losses, Less Than One Year	4.2	2.0
Fair Value, One Year or Greater	5.0	35.9
Unrealized Losses, One Year or Greater	0.1	0.5
Fair Value, Total	234.5	211.1
Unrealized Losses, Total	4.3	2.5
Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	76.0	33.3
Unrealized Losses, Less Than One Year	1.6	0.5
Fair Value, One Year or Greater	14.4	22.4
Unrealized Losses, One Year or Greater	0.8	0.6
Fair Value, Total	90.4	55.7
Unrealized Losses, Total	2.4	1.1
Fixed Maturities [Member] | Corporate Bonds [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	1,234.1	674.1
Unrealized Losses, Less Than One Year	27.6	9.9
Fair Value, One Year or Greater	23.1	77.7
Unrealized Losses, One Year or Greater	1.9	3.3
Fair Value, Total	1,257.2	751.8
Unrealized Losses, Total	29.5	13.2
Fixed Maturities [Member] | Commercial Mortgage-backed Securities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	133.1	58.2
Unrealized Losses, Less Than One Year	2.3	0.4
Fair Value, One Year or Greater	15.7	37.8
Unrealized Losses, One Year or Greater	1.0	1.1
Fair Value, Total	148.8	96.0
Unrealized Losses, Total	3.3	1.5
Fixed Maturities [Member] | Residential Mortgage-backed Securities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	317.6	164.4
Unrealized Losses, Less Than One Year	11.3	1.6
Fair Value, One Year or Greater	13.6	52.4
Unrealized Losses, One Year or Greater	0.7	1.1
Fair Value, Total	331.2	216.8
Unrealized Losses, Total	12.0	2.7
Fixed Maturities [Member] | Asset-backed Securities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	141.9	85.4
Unrealized Losses, Less Than One Year	1.0	0.4
Fair Value, One Year or Greater	6.3	31.9
Unrealized Losses, One Year or Greater	0.3	0.4
Fair Value, Total	148.2	117.3
Unrealized Losses, Total	1.3	0.8
Fixed Maturities [Member] | Collateralized Debt Obligations [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	192.7	34.6
Unrealized Losses, Less Than One Year	1.1	0.5
Fair Value, One Year or Greater	0.0	0.9
Unrealized Losses, One Year or Greater	0.0	0.0
Fair Value, Total	192.7	35.5
Unrealized Losses, Total	$ 1.1	$ 0.5