Summary of Reserves for Losses and LAE Activity (Detail) - USD ($) $ in Thousands	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Insurance [Abstract]
Net Reserves for Losses and LAE at Beginning of Year	$ 1,705,380	$ 1,510,451
Acquired Net Reserves	16,159
Provision for Losses and LAE for Claims Occurring in the Current Year	379,158	331,595
Increase (Decrease) in Estimated Losses and LAE for Claims Occurring in Prior Years	3,320	15,115
Incurred Losses and LAE	382,478	346,710
Losses and LAE Paid for Claims Occurring During:
Current Year	(32,832)	(47,235)
Prior Years	(292,843)	(249,895)
Losses and LAE Payments	(325,675)	(297,130)
Foreign Currency Adjustment	(3,310)	3,432
Net Reserves for Losses and LAE at End of Period	1,775,032	1,563,463
Reinsurance Recoverables on Unpaid Losses and LAE	799,084	782,864	$ 809,765
Gross Reserves for Losses and LAE at End of Period	$ 2,574,116	$ 2,346,327	$ 2,515,145

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The amount needed to reflect the estimated ultimate cost of settling claims relating to casualty insurance insured events that have occurred on or before a particular date (ordinarily, the balance sheet date) and the amount needed to provide for the estimated ultimate cost required to investigate and settle claims relating to insured events that have occurred on or before a particular date (ordinarily, the balance sheet date), whether or not reported to the insurer at that date.

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Amount of increase (decrease) in the cost of settling unpaid claims after deduction of reinsurance recoveries related to the acquisition of a business. Includes, but is not limited to, claims which have been incurred but not reported.

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Amount, after effects of reinsurance, of payments to settle claims incurred in the current period and related claims settlement costs.

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Amount, after effects of reinsurance, of payments to settle claims incurred in prior periods and related claims settlement costs.

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Amount of foreign currency gain (loss) which decreases (increases) the cost of settling unpaid claims after deduction of reinsurance recoveries. Includes, but is not limited to, claims which have been incurred but not reported.

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Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred and costs incurred in the claim settlement process.

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Liability as of the balance sheet date for amounts representing estimated cost of settling unpaid claims under the terms of the underlying insurance policies, less estimated reinsurance recoveries on such claims. This includes an estimate for claims which have been incurred but not reported. Claim adjustment expenses represent the costs estimated to be incurred in the settlement of unpaid claims.

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Amount, after effects of reinsurance, of payments to settle insured claims and pay costs incurred in the claims settlement process.

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Amount, after valuation allowance, recoverable under reinsurance contracts for losses reported to the ceding insurer but not yet paid and amounts expected for incurred losses and settlement expenses, which have not yet been reported to the ceding insurer.

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Amount, after effects of reinsurance, of expense for claims incurred in the current reporting period and related claims settlement costs.

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Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred in prior reporting periods and related claims settlement costs.

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