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 on ceded written premiums resulting from the change in the ceding rate.

+ References

No definition available.

+ Details

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

- 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 percentage which is credited to funds-held account.

+ References

No definition available.

+ Details

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

- Definition

Percentage points added to the reference rate to compute the variable rate on the funds held under reinsurance agreement.

+ References

No definition available.

+ Details

Name:	knsl_FundsHeldUnderReinsuranceAgreementsBasisSpreadonVariableRate
Namespace Prefix:	knsl_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

Name:	knsl_MultiLineQuotaShareReinsuranceAbstract
Namespace Prefix:	knsl_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the percent of the multi-line quota share treaty that was commuted.

+ References

No definition available.

+ Details

Name:	knsl_PercentageOfCommutation
Namespace Prefix:	knsl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- 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/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04.1)
-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 605
-Section 50
-Paragraph 1
-Subparagraph (b),(c)(2)
-URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 605
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=84167274&loc=d3e27175-158546

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 235
-Section S99
-Paragraph 2
-Subparagraph (SX 210.12-17)
-URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574992-122915

+ Details

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

- Definition

Amount of funds held as security for payments to be received under reinsurance contracts.

+ References

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

+ Details

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

- 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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 20
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=6480758&loc=d3e6811-158387

+ Details

Name:	us-gaap_ReinsuranceEffectOnClaimsAndBenefitsIncurredAmountCeded
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

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

- Details

Name:	us-gaap_ReinsurancePolicyTypeAxis=knsl_MultiLineQuotaShareReinsuranceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ReinsurancePolicyTypeAxis=knsl_MultiLineQuotaShareTreaty2013Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ReinsurancePolicyTypeAxis=knsl_MultiLineQuotaShareTreaty2014Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ReinsurancePolicyTypeAxis=knsl_MultiLineQuotaShareTreaty2015Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ReinsurancePolicyTypeAxis=knsl_MultiLineQuotaShareTreaty2016Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=knsl_TenYearUSTreasuryRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Reinsurance Multi-line Quota Share Reinsurance (Details) $ in Thousands	3 Months Ended	9 Months Ended		12 Months Ended
	Dec. 31, 2015 Rate	Sep. 30, 2016 Rate	Sep. 30, 2015 Rate	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) Rate	Dec. 31, 2015 USD ($) Rate	Jan. 01, 2017 USD ($)	Oct. 01, 2016 USD ($)	Jan. 01, 2016 USD ($)	Oct. 01, 2015 USD ($)	Jan. 01, 2015 USD ($) Rate	Dec. 31, 2014 USD ($) Rate
Reinsurance Retention Policy [Line Items]
Funds held for reinsurers				$ 0	$ 36,497
Multi-line Quota Share Reinsurance [Abstract]
Ceded earned premiums				33,373	47,031	$ 96,227
Ceded incurred losses and loss adjustment expenses				$ 12,200	13,700	41,200
Multi-line Quota Share Reinsurance [Member]
Multi-line Quota Share Reinsurance [Abstract]
Ceded earned premiums					16,996	67,950
Ceded incurred losses and loss adjustment expenses					4,380	30,978
Ceding commissions earned					11,936	34,254
Reinsurers' margin incurred					$ 680	$ 2,718
Multi-line Quota Share Reinsurance [Member] \| Maximum [Member]
Reinsurance Retention Policy [Line Items]
Loss ratio \| Rate					110.00%
Multi Line Quota Share Treaty - 2013 [Member]
Reinsurance Retention Policy [Line Items]
Effect of commutation											$ 11,900	$ 9,700
Commutation percent \| Rate											55.00%	45.00%
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 \| Rate						41.00%
Reinsurance margin \| Rate						4.00%
Ceding percentage \| Rate	40.00%		50.00%
Effect of change in ceding rate									$ 17,000	$ 6,800
Effect of commutation							$ 36,500
Multi Line Quota Share Treaty - 2015 [Member] \| Minimum [Member]
Reinsurance Retention Policy [Line Items]
Sliding scale commission \| Rate						25.00%
Multi Line Quota Share Treaty - 2015 [Member] \| Maximum [Member]
Reinsurance Retention Policy [Line Items]
Sliding scale commission \| Rate						41.00%
Multi Line Quota Share Treaty - 2016 [Member]
Reinsurance Retention Policy [Line Items]
Ceding percentage \| Rate		15.00%
Effect of commutation								$ 15,500
Ten Year U.S. Treasury Rate [Member]
Reinsurance Retention Policy [Line Items]
Funds-held account, interest rate percentage \| Rate					4.00%
Funds held under reinsurance agreements, basis spread on variable rate					150