Valuation and Qualifying Accounts (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for Uncollectable Reinsurance
Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	$ 12,078	$ 6,921	$ 11,332
Charged (Credited) to Costs and Expenses	519	5,157	(4,411)
Charged to Other Accounts	0	0	0
Deductions (Describe)	0	0	0
Ending Balance	12,597	12,078	6,921
Valuation Allowance in Deferred Taxes
Valuation and Qualifying Accounts Disclosure [Line Items]
Beginning Balance	699	721	776
Charged (Credited) to Costs and Expenses	1,099	(22)	(55)
Charged to Other Accounts	0	0	0
Deductions (Describe)	0	0	0
Ending Balance	$ 1,798	$ 699	$ 721