Valuation and Qualifying Accounts (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Allowance for uncollectible reinsurance recoverables
Valuation and Qualifying Accounts Disclosure [Line Items]
Charged to Other Accounts	$ 0.0	$ 0.0	$ 0.0
Allowance for Uncollectible Premiums Receivable
Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	0.4	0.5	0.4
Charged to Costs and Expenses	1.1	0.6	0.6
Charged to Other Accounts	0.0	0.0	0.0
Deductions	0.7	0.7	0.5
Ending Balance	$ 0.8	$ 0.4	$ 0.5