Investments - Schedule of Aging of Unrealized Losses on Company's Investments in Fixed Maturities, Equity Securities and Other Investments (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Unrealized losses one year or greater, less than	$ 0.1	$ 0.1
Fixed Maturities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	2,329.1	1,538.9
Unrealized Losses, Less Than One Year	54.4	17.2
Fair Value, One Year or Greater	102.5	342.7
Unrealized Losses, One Year or Greater	5.3	10.1
Fair Value, Total	2,431.6	1,881.6
Unrealized Losses, Total	59.7	27.3
Fixed Maturities [Member] | U.S. Governments [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	223.4	313.7
Unrealized Losses, Less Than One Year	6.1	1.9
Fair Value, One Year or Greater	17.2	83.7
Unrealized Losses, One Year or Greater	0.9	3.1
Fair Value, Total	240.6	397.4
Unrealized Losses, Total	7.0	5.0
Fixed Maturities [Member] | Foreign Governments [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	212.9	175.2
Unrealized Losses, Less Than One Year	5.9	2.0
Fair Value, One Year or Greater	5.0	35.9
Unrealized Losses, One Year or Greater	0.0	0.5
Fair Value, Total	217.9	211.1
Unrealized Losses, Total	5.9	2.5
Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	67.5	33.3
Unrealized Losses, Less Than One Year	1.1	0.5
Fair Value, One Year or Greater	14.6	22.4
Unrealized Losses, One Year or Greater	0.7	0.6
Fair Value, Total	82.1	55.7
Unrealized Losses, Total	1.8	1.1
Fixed Maturities [Member] | Corporate Bonds [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	1,195.0	674.1
Unrealized Losses, Less Than One Year	28.7	9.9
Fair Value, One Year or Greater	28.9	77.7
Unrealized Losses, One Year or Greater	1.8	3.3
Fair Value, Total	1,223.9	751.8
Unrealized Losses, Total	30.5	13.2
Fixed Maturities [Member] | Commercial Mortgage-backed Securities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	105.2	58.2
Unrealized Losses, Less Than One Year	1.9	0.4
Fair Value, One Year or Greater	15.8	37.8
Unrealized Losses, One Year or Greater	0.9	1.1
Fair Value, Total	121.0	96.0
Unrealized Losses, Total	2.8	1.5
Fixed Maturities [Member] | Residential Mortgage-backed Securities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	288.2	164.4
Unrealized Losses, Less Than One Year	8.9	1.6
Fair Value, One Year or Greater	14.0	52.4
Unrealized Losses, One Year or Greater	0.7	1.1
Fair Value, Total	302.2	216.8
Unrealized Losses, Total	9.6	2.7
Fixed Maturities [Member] | Asset-backed Securities [Member]
Schedule of Investments [Line Items]
Fair Value, Less Than One Year	107.1	85.4
Unrealized Losses, Less Than One Year	1.0	0.4
Fair Value, One Year or Greater	7.0	31.9
Unrealized Losses, One Year or Greater	0.3	0.4
Fair Value, Total	114.1	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	129.8	34.6
Unrealized Losses, Less Than One Year	0.8	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	129.8	35.5
Unrealized Losses, Total	$ 0.8	$ 0.5