Disclosures about Fair Value of Financial Instruments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Financial Instruments Owned and Pledged as Collateral [Line Items]
Premiums receivable	$ 371.7	$ 353.6	[1]
Reinsurance recoverables	987.3	997.2	[1]
Premiums receivable, allowance for doubtful accounts	4.5	5.2
Over 90 Days [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Premiums receivable	12.8	12.4
Reinsurance recoverables	6.8	9.9
Reinsurance recoverables, allowance for doubtful accounts	2.1	1.8
Premiums receivables, secured by collateral	0.3	0.3
Reinsurance recoverables, secured by collateral	$ 0.7	$ 0.4

[1] Derived from audited consolidated financial statements.