Refers to ceding commission earned during period.

No definition available.

Name:	knsl_CedingCommissionEarned
Namespace Prefix:	knsl_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Represents the percent of business ceded under the multi-line quota share treaty

+ References

No definition available.

+ Details

Name:	knsl_Cedingpercentage
Namespace Prefix:	knsl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the effect of the commutation of the multi-line quota share treaty on reinsurance recoverables on unpaid losses and receivables from reinsurers.

+ References

No definition available.

+ Details

Name:	knsl_Effectofcommutation
Namespace Prefix:	knsl_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Refers to interest rate which is credited to funds-held account.

+ References

No definition available.

+ Details

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Namespace Prefix:	knsl_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the provisional ceding commission rate on the multi-line quota share reinsurance treaty, which is set at the start of the contract and is subject to subsequent change.

+ References

No definition available.

+ Details

Name:	knsl_Provisionalcedingcommissionrate
Namespace Prefix:	knsl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the reinsurance margin on the multi-line quota share reinsurance treaty

+ References

No definition available.

+ Details

Name:	knsl_Reinsurancemargin
Namespace Prefix:	knsl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Refers to reinsurers' margin incurred during period.

+ References

No definition available.

+ Details

Name:	knsl_ReinsurersMarginIncurred
Namespace Prefix:	knsl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represent the sliding scale commission on the multi-line quota share reinsurance treaty, which varies inversely to the loss ratio.

+ References

No definition available.

+ Details

Name:	knsl_Slidingscalecommission
Namespace Prefix:	knsl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of earned premiums ceded to other entities.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 235
-Section S99
-Paragraph 2
-Subparagraph (SX 210.12-17(Column C))
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+ Details

Name:	us-gaap_CededPremiumsEarned
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Ratio of non-life losses and loss expenses to non-life net premiums earned.

+ References

No definition available.

+ Details

Name:	us-gaap_LossRatio
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of reduction of provision for policy benefits and costs incurred for policies ceded.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 20
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=99397103&loc=d3e6811-158387

+ Details

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

- Definition

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.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

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Namespace Prefix:
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Balance Type:
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- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
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- Details

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Data Type:	na
Balance Type:
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- Details

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Namespace Prefix:
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- Details

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Reinsurance Multi-line Quota Share Reinsurance (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2017	Oct. 01, 2016	Jan. 01, 2016
Multi-line Quota Share Reinsurance [Abstract]
Ceded earned premiums		$ 37,709	$ 33,373	$ 47,031
Ceded incurred losses and loss adjustment expenses		$ 25,500	$ 12,200	13,700
Multi-line Quota Share Reinsurance [Member]
Multi-line Quota Share Reinsurance [Abstract]
Ceded earned premiums				16,996
Ceded incurred losses and loss adjustment expenses				4,380
Ceding commissions earned				11,936
Reinsurers' margin incurred				$ 680
Multi-line Quota Share Reinsurance [Member] \| Maximum [Member]
Reinsurance Retention Policy [Line Items]
Loss ratio	110.00%
Multi Line Quota Share Treaty - 2014 [Member]
Reinsurance Retention Policy [Line Items]
Effect of commutation							$ 34,200
Multi Line Quota Share Treaty - 2015 [Member]
Reinsurance Retention Policy [Line Items]
Provisional ceding commission rate	41.00%
Reinsurance margin	4.00%
Effect of commutation					$ 36,500
Multi Line Quota Share Treaty - 2015 [Member] \| Minimum [Member]
Reinsurance Retention Policy [Line Items]
Sliding scale commission	25.00%
Multi Line Quota Share Treaty - 2015 [Member] \| Maximum [Member]
Reinsurance Retention Policy [Line Items]
Sliding scale commission	41.00%
Multi Line Quota Share Treaty - 2016 [Member]
Reinsurance Retention Policy [Line Items]
Ceding percentage	15.00%
Effect of commutation						$ 15,500
Ten Year U.S. Treasury Rate [Member]
Reinsurance Retention Policy [Line Items]
Funds Held Account Interest Rate	10 years