Reinsurance - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Reinsurance Disclosures [Abstract]
Allowance for uncollectible reinsurance balances receivable on paid losses and incurred claims	$ 3.7	$ 4.4
Amount of collateral under reinsurance agreement	154.2	252.3
Paid loss recoverables in insurance recoverables	140.8	208.3
Losses and loss adjustment expenses	742.0	747.6	863.1
Net of amounts ceded to reinsurers	$ 292.8	$ 223.6	$ 290.0