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Commitments and Contingencies - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
reinsurance contract obligation [Line Items]
Minimum commitment under noncancelable purchase obligations	$ 546.3
unrecorded unconditional purchase obligations reinsurance [Member] | Lower Limit
reinsurance contract obligation [Line Items]
Reinsurance Contract Terms	1 year
Other Commitment	$ 90.7
unrecorded unconditional purchase obligations reinsurance [Member] | Upper Limit
reinsurance contract obligation [Line Items]
Reinsurance Contract Terms	3 years

X

- Definition

Period Terms of Multiple Layer Property Catastrophe excess of loss reinsurance claims with various reinsurers

+ References

No definition available.

+ Details

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X

- Definition

[Line Items] for reinsurance contract obligation [Table]

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No definition available.

+ Details

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X

- Definition

Minimum amount of other commitment not otherwise specified in the taxonomy. Excludes commitments explicitly modeled in the taxonomy, including but not limited to, long-term and short-term purchase commitments, recorded and unrecorded purchase obligations, supply commitments, registration payment arrangements, leases, debt, product warranties, guarantees, environmental remediation obligations, and pensions.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Amount of the unrecorded obligation to transfer funds in the future for fixed or minimum amounts or quantities of goods or services at fixed or minimum prices (for example, as in take-or-pay contracts or throughput contracts).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 440
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=77896938&loc=d3e25336-109308

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X

- Details

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