Reinsurance - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Effects of Reinsurance [Line Items]
Reinsurance recoverable supported by comfort trust	$ 6,501,700	$ 6,280,337
Deposit receivable	1,111,571	1,199,537
Modified coinsurance payable	$ 1,111,571	$ 1,199,537
Sales Revenue \| Credit Concentration Risk \| Non-affiliates \| Rated A- or better
Effects of Reinsurance [Line Items]
Reinsurance recoverable	99.80%	99.80%
Sales Revenue \| Credit Concentration Risk \| Non-affiliates \| Rated A- or better \| Comfort Trust [Member]
Effects of Reinsurance [Line Items]
Reinsurance recoverable supported by comfort trust	$ 5,900,000	$ 5,900,000
Sales Revenue \| Credit Concentration Risk \| Non-affiliates \| Rated A- or better \| Allstate Life Insurance Company
Effects of Reinsurance [Line Items]
Reinsurance recoverable	73.00%	75.00%

The consideration due to the assuming enterprise as of the balance sheet date under the terms of the reinsurance contract in exchange for the right to receive reimbursement from such entity for covered claims and expenses.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.13(c))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

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Carrying value of amounts transferred to third parties for security purposes that are expected to be returned or applied towards payment in the future.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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No definition available.

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Amount, after valuation allowance, recoverable under reinsurance contracts including premium paid under reinsurance contracts. Examples include, but are not limited to, settled and unsettled claims, incurred but not reported losses, loss adjustment expense, premium paid, policy benefits and policy reserves.

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