Investments - Schedule of Amortized Cost, Gross Unrealized Gains, Gross Unrealized Losses and Fair Value of Investments (Detail) - USD ($) $ in Millions		Sep. 30, 2016	Dec. 31, 2015
Schedule of Invested Assets [Line Items]
Total investments		$ 4,313.6	$ 4,115.7	[1]
Fixed Maturities [Member]
Schedule of Invested Assets [Line Items]
Invested Assets Amortized Cost		3,008.8	2,971.0
Investments Gross Unrealized Gains		72.5	43.1
Investments Gross Unrealized losses		36.7	86.8
Total investments		3,044.6	2,927.3
Fixed Maturities [Member] | CLO [Member]
Schedule of Invested Assets [Line Items]
Invested Assets Amortized Cost	[2]	150.6	137.5
Investments Gross Unrealized Gains	[2]	1.0	0.1
Investments Gross Unrealized losses	[2]	0.4	1.7
Total investments	[2]	151.2	135.9
Fixed Maturities [Member] | U.S. Governments [Member]
Schedule of Invested Assets [Line Items]
Invested Assets Amortized Cost		216.4	207.9
Investments Gross Unrealized Gains		1.8	0.7
Investments Gross Unrealized losses		0.2	0.7
Total investments		218.0	207.9
Fixed Maturities [Member] | Non-U.S. Governments [Member]
Schedule of Invested Assets [Line Items]
Invested Assets Amortized Cost		92.7	93.0
Investments Gross Unrealized Gains		0.8	0.1
Investments Gross Unrealized losses		0.0	1.3
Total investments		93.5	91.8
Fixed Maturities [Member] | Obligations of States and Political Subdivisions [Member]
Schedule of Invested Assets [Line Items]
Invested Assets Amortized Cost		387.0	467.6
Investments Gross Unrealized Gains		20.8	20.7
Investments Gross Unrealized losses		0.1	0.3
Total investments		407.7	488.0
Fixed Maturities [Member] | Credit-Financial [Member]
Schedule of Invested Assets [Line Items]
Invested Assets Amortized Cost		494.3	502.3
Investments Gross Unrealized Gains		11.4	6.2
Investments Gross Unrealized losses		0.5	2.6
Total investments		505.2	505.9
Fixed Maturities [Member] | Credit-Industrial [Member]
Schedule of Invested Assets [Line Items]
Invested Assets Amortized Cost		627.5	555.8
Investments Gross Unrealized Gains		16.1	5.3
Investments Gross Unrealized losses		2.5	12.4
Total investments		641.1	548.7
Fixed Maturities [Member] | Credit-Utility [Member]
Schedule of Invested Assets [Line Items]
Invested Assets Amortized Cost		150.0	168.1
Investments Gross Unrealized Gains		6.2	1.0
Investments Gross Unrealized losses		1.8	13.6
Total investments		154.4	155.5
Fixed Maturities [Member] | CMO/MBS-Agency [Member]
Schedule of Invested Assets [Line Items]
Invested Assets Amortized Cost	[3]	162.9	126.5
Investments Gross Unrealized Gains	[3]	5.3	5.0
Investments Gross Unrealized losses	[3]	0.1	0.5
Total investments	[3]	168.1	131.0
Fixed Maturities [Member] | CMO/MBS-Non Agency [Member]
Schedule of Invested Assets [Line Items]
Invested Assets Amortized Cost		5.6	6.7
Investments Gross Unrealized Gains		0.5	0.6
Investments Gross Unrealized losses		0.0	0.0
Total investments		6.1	7.3
Fixed Maturities [Member] | CMBS [Member]
Schedule of Invested Assets [Line Items]
Invested Assets Amortized Cost	[4]	203.6	186.5
Investments Gross Unrealized Gains	[4]	2.0	0.5
Investments Gross Unrealized losses	[4]	0.2	1.6
Total investments	[4]	205.4	185.4
Fixed Maturities [Member] | ABS [Member]
Schedule of Invested Assets [Line Items]
Invested Assets Amortized Cost	[5]	122.5	111.0
Investments Gross Unrealized Gains	[5]	0.5	0.2
Investments Gross Unrealized losses	[5]	0.2	0.7
Total investments	[5]	122.8	110.5
Fixed Maturities [Member] | Foreign Denominated: Governments [Member]
Schedule of Invested Assets [Line Items]
Invested Assets Amortized Cost		165.7	168.8
Investments Gross Unrealized Gains		1.8	0.8
Investments Gross Unrealized losses		9.8	19.3
Total investments		157.7	150.3
Fixed Maturities [Member] | Foreign Denominated: Credit [Member]
Schedule of Invested Assets [Line Items]
Invested Assets Amortized Cost		123.1	130.2
Investments Gross Unrealized Gains		1.5	1.1
Investments Gross Unrealized losses		13.3	20.4
Total investments		111.3	110.9
Fixed Maturities [Member] | Foreign denominated: ABS/CMBS [Member]
Schedule of Invested Assets [Line Items]
Invested Assets Amortized Cost		18.1	19.7
Investments Gross Unrealized Gains		0.0	0.1
Investments Gross Unrealized losses		2.8	2.2
Total investments		15.3	17.6
Fixed Maturities [Member] | Foreign denominated: CLO [Member]
Schedule of Invested Assets [Line Items]
Invested Assets Amortized Cost		88.8	89.4
Investments Gross Unrealized Gains		2.8	0.7
Investments Gross Unrealized losses		4.8	9.5
Total investments		86.8	80.6
Equity Securities [Member]
Schedule of Invested Assets [Line Items]
Invested Assets Amortized Cost		345.6	349.7
Investments Gross Unrealized Gains		115.4	131.5
Investments Gross Unrealized losses		7.3	17.3
Total investments		453.7	463.9
Other Investments [Member]
Schedule of Invested Assets [Line Items]
Invested Assets Amortized Cost		518.7	506.9
Investments Gross Unrealized Gains		7.5	6.9
Investments Gross Unrealized losses		0.1	0.1
Total investments		526.1	513.7
Short-Term Investments [Member]
Schedule of Invested Assets [Line Items]
Invested Assets Amortized Cost		289.2	211.2
Investments Gross Unrealized Gains		0.0	0.0
Investments Gross Unrealized losses		0.0	0.4
Total investments		289.2	210.8
Securities and Non Securities Investment [Member]
Schedule of Invested Assets [Line Items]
Invested Assets Amortized Cost		4,162.3	4,038.8
Investments Gross Unrealized Gains		195.4	181.5
Investments Gross Unrealized losses		44.1	104.6
Total investments		$ 4,313.6	$ 4,115.7

[1] Derived from audited consolidated financial statements.

[2] Collateralized loan obligations (“CLO”).

[3] Collateralized mortgage obligations/mortgage-backed securities (“CMO/MBS”).

[4] Commercial mortgage-backed securities (“CMBS”).

[5] Asset-backed securities (“ABS”).