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Valuation and Qualifying Accounts (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Allowance for uncollectible reinsurance recoverables      
Valuation and Qualifying Accounts Disclosure [Line Items]      
Beginning Balance $ 0.0 $ 0.0 $ 0.0
Charged to Costs and Expenses 0.0 0.0 0.0
Charged to Other Accounts 0.0 0.0 0.0
Deductions 0.0 0.0 0.0
Ending Balance 0.0 0.0 0.0
Allowance for Uncollectible Premiums Receivable      
Valuation and Qualifying Accounts Disclosure [Line Items]      
Beginning Balance 0.5 1.0 0.8
Charged to Costs and Expenses 0.7 0.8 1.2
Charged to Other Accounts 0.0 0.0 0.0
Deductions 0.8 1.3 1.0
Ending Balance $ 0.4 $ 0.5 $ 1.0