Employee Retirement Benefit Plans - Additional Information (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Parent Company
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ 34.1	$ 30.2
Fair value of plan assets	3.4	2.4
Transatlantic Holdings Incorporated
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	71.8	65.4
Fair value of plan assets	52.4	$ 48.7
WWSC Holdings, LLC
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	23.2
Fair value of plan assets	$ 14.0

Amount of actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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- Definition

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+ References

No definition available.

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- Definition

Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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