Reserves for Losses and Loss Adjustment Expenses - Summary of Information about Incurred and Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance, Commercial Specialty Casualty Products (Detail) $ in Millions		Dec. 31, 2016 USD ($) Claim	Dec. 31, 2015 USD ($)	[1]	Dec. 31, 2014 USD ($)	[1]	Dec. 31, 2013 USD ($)	[1]	Dec. 31, 2012 USD ($)	[1]	Dec. 31, 2011 USD ($)	[1]
Claims Development [Line Items]
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance		$ 2,131.6
Commercial Specialty [Member] | Casualty Products [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		758.2
Cumulative Paid Losses & ALAE, Net of Reinsurance		454.3
Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance		70.3
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance		374.2
Commercial Specialty [Member] | Casualty Products [Member] | Accident Year 2011 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		154.0	$ 158.2		$ 157.5		$ 159.0		$ 155.1		$ 140.6
Cumulative Paid Losses & ALAE, Net of Reinsurance		135.1	126.1		111.3		85.9		57.5		$ 23.1
IBNR & Expected Development on Reported Claims		$ 9.1
Cumulative Number of Reported Claims | Claim	[2]	28,462
Commercial Specialty [Member] | Casualty Products [Member] | Accident Year 2012 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 151.5	152.2		149.7		146.3		140.3
Cumulative Paid Losses & ALAE, Net of Reinsurance		120.8	105.3		80.7		51.0		$ 20.1
IBNR & Expected Development on Reported Claims		$ 15.3
Cumulative Number of Reported Claims | Claim	[2]	24,078
Commercial Specialty [Member] | Casualty Products [Member] | Accident Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 133.2	135.1		133.2		126.6
Cumulative Paid Losses & ALAE, Net of Reinsurance		93.6	73.6		49.4		$ 18.9
IBNR & Expected Development on Reported Claims		$ 16.9
Cumulative Number of Reported Claims | Claim	[2]	19,117
Commercial Specialty [Member] | Casualty Products [Member] | Accident Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 116.9	118.6		115.5
Cumulative Paid Losses & ALAE, Net of Reinsurance		58.7	38.4		$ 17.4
IBNR & Expected Development on Reported Claims		$ 29.1
Cumulative Number of Reported Claims | Claim	[2]	16,577
Commercial Specialty [Member] | Casualty Products [Member] | Accident Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 106.7	102.6
Cumulative Paid Losses & ALAE, Net of Reinsurance		35.0	$ 16.4
IBNR & Expected Development on Reported Claims		$ 40.5
Cumulative Number of Reported Claims | Claim	[2]	13,758
Commercial Specialty [Member] | Casualty Products [Member] | Accident Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 95.9
Cumulative Paid Losses & ALAE, Net of Reinsurance		11.1
IBNR & Expected Development on Reported Claims		$ 55.6
Cumulative Number of Reported Claims | Claim	[2]	9,720

[1] Information presented for calendar years prior to 2016 is required supplementary information and is unaudited.

[2] The cumulative number of reported claims is measured by individual claimant at a coverage level. Reported occurrences that do not result in a liability are included as reported claims.