Reserves for Losses and Loss Adjustment Expenses - Summary of Information about Incurred and Cumulative Paid Losses and Allocated Loss Adjustment Expenses, Net of Reinsurance, International Specialty Property 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
International Specialty [Member] | Property Products [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		278.1
Cumulative Paid Losses & ALAE, Net of Reinsurance		219.0
Outstanding liabilities for unpaid losses and ALAE prior to 2011, net of reinsurance		4.4
Total outstanding liabilities for unpaid losses and ALAE, net of reinsurance		63.5
International Specialty [Member] | Property Products [Member] | Accident Year 2011 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		102.1	$ 102.2		$ 102.9		$ 105.0		$ 102.9		$ 114.9
Cumulative Paid Losses & ALAE, Net of Reinsurance		97.2	96.0		93.7		86.2		65.1		$ 39.9
IBNR & Expected Development on Reported Claims		$ 0.0
Cumulative Number of Reported Claims | Claim	[2]	400
International Specialty [Member] | Property Products [Member] | Accident Year 2012 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 46.5	51.7		50.5		51.5		47.3
Cumulative Paid Losses & ALAE, Net of Reinsurance		44.1	49.7		40.7		31.2		$ 12.4
IBNR & Expected Development on Reported Claims		$ 0.9
Cumulative Number of Reported Claims | Claim	[2]	299
International Specialty [Member] | Property Products [Member] | Accident Year 2013 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 33.3	35.1		35.7		33.9
Cumulative Paid Losses & ALAE, Net of Reinsurance		30.8	28.5		18.7		$ 4.9
IBNR & Expected Development on Reported Claims		$ 0.8
Cumulative Number of Reported Claims | Claim	[2]	351
International Specialty [Member] | Property Products [Member] | Accident Year 2014 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 26.2	28.2		28.0
Cumulative Paid Losses & ALAE, Net of Reinsurance		20.3	13.7		$ 3.3
IBNR & Expected Development on Reported Claims		$ 2.6
Cumulative Number of Reported Claims | Claim	[2]	339
International Specialty [Member] | Property Products [Member] | Accident Year 2015 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 24.5	28.2
Cumulative Paid Losses & ALAE, Net of Reinsurance		11.8	$ 4.6
IBNR & Expected Development on Reported Claims		$ 5.5
Cumulative Number of Reported Claims | Claim	[2]	443
International Specialty [Member] | Property Products [Member] | Accident Year 2016 [Member]
Claims Development [Line Items]
Incurred Losses & ALAE, Net of Reinsurance		$ 45.5
Cumulative Paid Losses & ALAE, Net of Reinsurance		14.8
IBNR & Expected Development on Reported Claims		$ 19.5
Cumulative Number of Reported Claims | Claim	[2]	742

[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.