Segment Information - Additional Information (Detail)	12 Months Ended
Dec. 31, 2018 Segment
Segment Reporting Information [Line Items]
Number of underwriting Segments	3
Marsh & McLennan Companies Inc. \| Gross Premiums Written
Segment Reporting Information [Line Items]
Concentration percentage of gross premiums written	10.00%
U.S.Insurance
Segment Reporting Information [Line Items]
Number of Operating Segments	3
International Insurance
Segment Reporting Information [Line Items]
Number of Operating Segments	3

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.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
-Topic 275
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Data Type:	num:percentItemType
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