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, 2017		Dec. 31, 2016
Schedule Of Investments [Line Items]
Fair Value, Less Than One Year		$ 1,426.3		$ 1,365.2
Unrealized Losses, Less Than One Year		28.3		50.9
Fair Value, One Year or Greater		55.0		89.0
Unrealized Losses, One Year or Greater		2.3		3.2
Fair Value, Total		1,481.3		1,454.2
Unrealized Losses, Total		30.6		54.1
Fixed Maturities [Member]
Schedule Of Investments [Line Items]
Fair Value, Less Than One Year		1,342.1		1,298.0
Unrealized Losses, Less Than One Year		23.0		45.1
Fair Value, One Year or Greater		55.0		89.0
Unrealized Losses, One Year or Greater		2.3		3.2
Fair Value, Total		1,397.1		1,387.0
Unrealized Losses, Total		25.3		48.3
Fixed Maturities [Member] | Collateralized Loan Obligations [Member]
Schedule Of Investments [Line Items]
Fair Value, Less Than One Year		63.1		122.5
Unrealized Losses, Less Than One Year		1.7		8.6
Fair Value, One Year or Greater		3.0		16.9
Unrealized Losses, One Year or Greater		0.1		0.5
Fair Value, Total		66.1		139.4
Unrealized Losses, Total		1.8		9.1
Fixed Maturities [Member] | Residential Mortgage-backed Securities [Member]
Schedule Of Investments [Line Items]
Fair Value, Less Than One Year		110.9		87.5	[1]
Unrealized Losses, Less Than One Year		1.6		1.7	[1]
Fair Value, One Year or Greater		6.2		4.4	[1]
Unrealized Losses, One Year or Greater		0.1		0.0	[1]
Fair Value, Total		117.1		91.9	[1]
Unrealized Losses, Total		1.7		1.7	[1]
Fixed Maturities [Member] | U.S. Governments [Member]
Schedule Of Investments [Line Items]
Fair Value, Less Than One Year		267.1		183.4	[1]
Unrealized Losses, Less Than One Year		3.1		4.5	[1]
Fair Value, One Year or Greater		2.7		0.0	[1]
Unrealized Losses, One Year or Greater		0.1		0.0	[1]
Fair Value, Total		269.8		183.4	[1]
Unrealized Losses, Total		3.2		4.5	[1]
Fixed Maturities [Member] | Foreign Governments [Member]
Schedule Of Investments [Line Items]
Fair Value, Less Than One Year		183.8		201.2
Unrealized Losses, Less Than One Year		3.6		8.0
Fair Value, One Year or Greater		0.0		0.0
Unrealized Losses, One Year or Greater		0.0		0.0
Fair Value, Total		183.8		201.2
Unrealized Losses, Total		3.6		8.0
Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]
Schedule Of Investments [Line Items]
Fair Value, Less Than One Year	[2]	42.1		72.6
Unrealized Losses, Less Than One Year	[2]	0.8		1.7
Fair Value, One Year or Greater	[2]	1.8		1.8
Unrealized Losses, One Year or Greater	[2]	0.0		0.1
Fair Value, Total	[2]	43.9		74.4
Unrealized Losses, Total	[2]	0.8		1.8
Fixed Maturities [Member] | Corporate Bonds [Member]
Schedule Of Investments [Line Items]
Fair Value, Less Than One Year		523.1		490.5
Unrealized Losses, Less Than One Year		10.4		17.7
Fair Value, One Year or Greater		32.9		50.6
Unrealized Losses, One Year or Greater		1.5		1.8
Fair Value, Total		556.0		541.1
Unrealized Losses, Total		11.9		19.5
Fixed Maturities [Member] | Commercial Mortgage-backed Securities [Member]
Schedule Of Investments [Line Items]
Fair Value, Less Than One Year		83.7		70.6
Unrealized Losses, Less Than One Year		1.3		1.5
Fair Value, One Year or Greater		4.9		7.1
Unrealized Losses, One Year or Greater		0.0		0.1
Fair Value, Total		88.6		77.7
Unrealized Losses, Total		1.3		1.6
Fixed Maturities [Member] | Asset-backed Securities [Member]
Schedule Of Investments [Line Items]
Fair Value, Less Than One Year		68.3		69.7
Unrealized Losses, Less Than One Year		0.5		1.4
Fair Value, One Year or Greater		3.5		8.2
Unrealized Losses, One Year or Greater		0.5		0.7
Fair Value, Total		71.8		77.9
Unrealized Losses, Total		1.0		2.1
Equity Securities [Member]
Schedule Of Investments [Line Items]
Fair Value, Less Than One Year		52.3		62.1
Unrealized Losses, Less Than One Year		5.2		5.7
Fair Value, One Year or Greater		0.0		0.0
Unrealized Losses, One Year or Greater		0.0		0.0
Fair Value, Total		52.3		62.1
Unrealized Losses, Total		5.2		5.7
Other Investments [Member]
Schedule Of Investments [Line Items]
Fair Value, Less Than One Year		0.3		0.3
Unrealized Losses, Less Than One Year		0.1		0.1
Fair Value, One Year or Greater		0.0		0.0
Unrealized Losses, One Year or Greater		0.0		0.0
Fair Value, Total		0.3		0.3
Unrealized Losses, Total		0.1		0.1
Short-Term Investments [Member]
Schedule Of Investments [Line Items]
Fair Value, Less Than One Year		31.6	[2]	4.8
Unrealized Losses, Less Than One Year		0.0	[2]	0.0
Fair Value, One Year or Greater		0.0	[2]	0.0
Unrealized Losses, One Year or Greater		0.0	[2]	0.0
Fair Value, Total		31.6	[2]	4.8
Unrealized Losses, Total		$ 0.0	[2]	$ 0.0

[1] Unrealized losses one year or greater are less than $0.1 million.

[2] Unrealized losses less than one year are less than $0.1 million.